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STEREOCHEMISTRY
In the previous chapters we discussed electron distribution in organic molecules. In this chapter we discuss the three-dimensional structure of organic compounds.' Tie struc-
ture may be such that stereoisomerism2 is possible. Stereoisomers are compounds made
up of the same atoms bonded by the same sequence of bonds but having different
three-dimensional structures which are not interchangeable. These three-dimensional struc-
tures are (^) called (^) configurations.
OPTICAL ACTIVITY AND CHIRALITY
Any material^ that^ rotates^ the^ plane of^ polarized light is said to (^) be (^) optically active. If (^) a (^) pure compound is optically active, the molecule is nonsuperimposable on its mirror image. If a molecule is (^) superimposable on its (^) mirror (^) image, the (^) compound does (^) not (^) rotate (^) the (^) plane of (^) polarized (^) light; it is (^) optically inactive. The (^) property of (^) nonsuperimposability of an on its (^) mirror object image is^ called^ chirality. If^ a^ molecule^ is^ not^ superimposable on (^) its mirroor image, it is^ chiral.^ If^ it^ is^ superimposable on^ its^ mirror^ image, it (^) is (^) achiral. (^) The (^) relationship between (^) optical activity and (^) chirality is (^) absolute. No (^) exceptions are (^) known, and (^) many thousands of^ cases^ have^ been^ found^ in accord^ with it (^) (however, see (^) p. 98). The (^) ultimate criterion, then,^ for^ optical activity is^ chirality (^) (nonsuperimposability on (^) the (^) mirror (^) image). This is^ both^ a^ necessary and^ a^ sufficient^ condition. This (^) fact (^) has (^) been (^) used (^) as (^) evidence (^) for the structure determination of^ many (^) compounds, and (^) historically the (^) tetrahedral (^) nature of carbon was^ deduced^ from^ the^ hypothesis that^ the^ relationship might be (^) true. If a (^) molecule is (^) nonsuperimposable on its (^) mirror (^) image, the (^) mirror image must^ be a different molecule, since^ superimposability 1s^ the^ same^ as (^) identity. In (^) each (^) case (^) of (^) optical activity of^ a^ pure compound there^ are^ two^ and^ oniy twO^ ISomers, called (^) enantiomers (^) (some- times enantiomorphs), which^ differ^ in^ structure^ only in^ the (^) lett- (^) and (^) right-handedness of
AND CHIRALITY 95
FIGURE 4.1 Enantiomers.
their orientations (Figure 4.1). Enantiomers have
except in^ two^ identical^ physical^ and^ chemical^ properties important (^) respects:
- (^) They rotate (^) the (^) plane of
polarized light in^ opposite directions, though in equal
amounts. The isomer that rotates the
isomer and plane^ to^ the^ left^ (counterclockwise)^ is^ called^ the^ levo
is designated (), while the one that rotates
the plane to the right (clockwise)
iscalled the^ dextro^ isomer (^) and (^) is (^) designated (+). Because are (^) often (^) called they^ differ^ in^ this^ property (^) they optical (^) antipodes.
- (^) They react at (^) different (^) rates (^) with (^) other chiral
together that^ the^ distinction compounds.^ These^ rates^ may^ be^ so^ close
is practically useless, or they
enantiomer may^ be^ so^ far^ apart^ that^ one
undergoes the^ reaction^ at^ a convenient rate while the other does not
all. This s the reason that react^ at
many compounds are biologically active while their
are (^) not. (^) Enantiomers (^) react at (^) the (^) same rate enantiomers
with achiral compounds.a
In (^) general, it (^) may be (^) said that (^) enantiomers have
identical properties in a
environment, but^ their properties may differ in an symmetrical
the (^) important differences unsymmetrical^ environment.^ Besides
previously noted, enantiomers may react at different rates with
achiral molecules if an optically active
in (^) an catalyst^ is^ present;^ they^ may have^ different (^) solubilities optically active^ solvent; (^) they may have (^) different (^) indexes of spectra when (^) examined with refraction^ or^ absorption
circularly polarized light, etc. In most cases these
are (^) too (^) small (^) to be (^) useful and are (^) often too (^) small to (^) be differences measured.
Aithough pure compounds are always optically active if
molecules, mixtures of they^ are^ composed^ of^ chiral equal amounts^ of^ enantiomers are (^) optically inactive (^) since and (^) opposite rotations (^) cancel. Such (^) mixtures are the (^) equal called (^) racemic (^) mixtures° or Their (^) properties (^) are (^) not (^) always (^) the (^) same (^) as (^) those of (^) the racemates.
erties in the gaseous or individual^ enantiomers.^ The prop-
liquid (^) state or^ in^ solution (^) usually are (^) the (^) same,
nearly ideal, but properties since^ such^ a^ mixture
involving the^ solid state, such as melting points,
a (^) heats of (^) fusion,are often (^) different. (^) Thus solubilities, 4-206°C (^) and a racemie^ tartaric^ acid^ has.a^ melting (^) pont o solubility in (^) water at^ 20°C of (^206) g/liter, (^) while
for the (+) or the (-
nteractions between (^) electrons, nucleons, and (^) certain (^) components of (^) nucleons
Os,interaat' violate parity; that is, mirror imarr interactions (e.g., bosons). called weak inter.
98 STEREOCHEMISTRY
However, the amount of rotation is greatly dependent on the nature of the four groups, in general increasing with increasing differences in polarizabilities among the groups. Alkyl groups have^ very similar^ polarizabilities and^ the^ optical^ activity^ of^ 5-ethyl-5-propylundecane IS too low to be measureable at any wavelength between 280 and 580 nm.
- (^) Compounds with other (^) quadrivalent chiral atoms.22^ Any molecule^ containing^ an^ atom that has four bonds (^) pointing to the corners^ of^ a^ tetrahedron^ will^ be^ optically^ active^ if^ the four groups are different. Among atoms in this category are Si,2" Ge, Sn,24 and N (in
quaternary salts^ or^ N-oxides),25^ In^ sulfones^ the^ sulfur^ bonds^ tetrahedrally,^
but since two of the (^) groups are (^) always oxygen, no^ chirality normally results.^ However,^ the^ preparation26^ of
PhCH-0-$-"o 0-P-OR
-CHS -CH
an optically active sulfone (2) in which one oxygen is 150 and the other 10 illustrates the
point that^ slight differences^ in^ groups^ are^ all^ that^ is^ necessary.^ This^ has^ been^ taken^ even
further with the preparation of the ester 3, both enantiomers of which have been prepared."
Optically active chiral phosphates 4 have similarly been made
3. Compounds with tervalent chiral atoms. Atoms with pyramidal bonding might be
expected to give rise to opticai activity if the atom is connected to three different groups, since the unshared pair of electrons is analogous to a fourth group, necessarily different from the others. For example, a secondary or tertiary amine where X, Y, and Z are different
--Z
would be expected to be chiral and thus resolvable. Many attempts have been made to resolve such compounds, but until 1968 all of them failed because of pyramidal inversion, which is a rapid oscillation of the unshared pair from one side of the XYZ plane to the other, thus converting the molecule into its enantiomer.30 For ammonia there are 2 x 101
RS
CHAPTER 4 OPTICAL ACTIVITY^ AND^ CHIRALITY^99
inversions every second. The inversion is less rapid in substituted ammonias (amines, amides, etc.). Two types of nitrogen atom invert particularly slowly, namely, a nitrogen
atom in^ a^ three-membered^ ring and^ a^ nitrogen atom^ connected^ to^ another^ atom^ bearing^ an
unshared pair. Even in such compounds, however, for many years pyramidal inversion
proved too rapid to permit isolation of separate isomers. This goal was accomplished" only when compounds were synthesized in which both features are combined: a nitrogen atom
in a three-membered ring connected to an atom containing an unshared pair. For example,
the two isomers of 1-chloro-2-methylaziridine (5 and 6) were separated and do not inter convert at^ room^ temperature.32 In^ suitable^ cases^ this^ barrier^ to^ inversion^ can^ result in compounds that^ are^ optically^ active^ solely^ because^ of^ a^ chiral^ tervalent^ nitrogen^ atom.^ For
Mirror
CI H H^ CI^ Me^ Me Ci
Me CI^ Me Me^ Me trans cis 5 6
COOEt
N-H
EtOOC N
OMe H 8 9
OMe
H COOMe
NC- -CGOMe^ Me0OCCH,CMeN-OCH,Ph N^11 o
OMe
10
itrogen is^ connected^ to^ an^ atom^
with an^ unshared^ pair.^ Conformational^ stability^
has
Deen demonstrated^ for^ oxaziridines,"^
diaziridines, (^) e.g., 8,"^ triaziridines,^ e.g.^ 9,5^ and
Admple, 7 has^ been^ resolved^ into^ its^ separate^
enantiomers.a Note^ that^ in^ this^ case^ to0, th
CRS
CHAPTER (^4) OPTICAL ACTIVITY AND CHIRALITY 101
B FIGURE^ 4.2^ Perpendicular disymmetric planes.
resolved.4 This type of molecule is a kind of expanded tetrahedron and has the same
symmetry5. Restricted properties rotation^ as^ any giving^ other rise^ tetrahedron. to perpendicular disymmetric planes. Certain com-
pounds that^ do^ not^ contain^ asymmetric^ atoms^ are^ nevertheless^ chiral^ because^ they^ contain a structure^ that^ can^ be^ schematically represented as^ in^ Figure^ 4.2.^ For^ these^ compounds^ we can draw two perpendicular planes neither of which can be bisected by a plane of symmetry. If either^ plane could^ be^ so^ bisected,^ the^ molecule^ would^ be^ superimposable^ on^ its^ mirror
image, since^ such^ a^ plane^ would^ be^ a^ plane^ of^ symmetry.^ These^ points^
will be^ illustrated by examples. Biphenyls containing^ four^ large^ groups^ in^ the^ ortho^ positions^
cannot (^) freely rotate^ about
the central^ bond^ because^ of^ steric^ hindrance.45^ In^ such^ compounds^ the^ two^ rings^
are in
perpendicular planes.^ If^ either^ ring^ is^ symmetrically^ substituted,^
the molecule^ has^ a^ plane
of (^) symmetry. For^ example, consider:
0N CI CI COOH
HOOCS
NO,
COOH
O,N Mirror
Ring B^ is^ symmetrically^
substituted. A^ plane drawn^ perpendicular^ to^ ring^ B^ contains^ all^
the
atoms and^ groups in^ ring^ A;^ hence^ it^
iS (^) a (^) plane of^ symmetry and^ the^ compound^ is^ achiral.
On the other hand, consider:
NO
HOOC
NO COOH
COOH N0 O,N^ HOOC Mirror
resolved. Note^ that^ groups^ in^ the^ para^ position^
cannot cause^ lack^ of^ symmetry.^ Isomers^
that
L Chem SeG. 1969, 91,
There is no^ plane of^ symmetry^
and the molecule^ is^ chiral;^ many^ such^ compounds^ have^ been
CHAPTER 4 CRS UPTICAL (^) ACTIVITY AND (^) CHIRALITY (^103)
Like (^) biphenyls, allenes^ are^ chiral (^) only if (^) both sides (^) are example, unsymmetrically^ substituted.50^ For
CH C=C=C
CH
CH =C=C
,H CH H
=C= H (^) H H CH,
Inactive (^) Inactive (^) Active
These cases are completely different from the cis-trans isomerism of compounds with one double bond (p. 127). In the latter cases the four groups are all in one plane, the isomers
are not^ enantiomers, and^ neither (^) is (^) chiral, while in (^) allenes the (^) groups are in two (^) perpendicular planes and the isomers are a pair of optically active enantiomers. When three, five, or any odd number of cumulative double bonds exist, orbital overlap causes the four groups to occupy one plane and cis-trans isomerism is observed. When four, six, or any even number of cumulative double bonds exist, the situation is analogous to that in the allenes and optical activity is possible. 16 has been resolved. Among other types of compounds that contain the system illustrated in Figure 4.2 and
(CH,),C C(CH) CC=C=
O)
CI
NH (^) NH H H H CH (^) CH H 18 17
that (^) are similarly chiral^ if^ both^ sides^
are dissymmetric^ are^ spiranes,^ e.g.,^ 17,^
and compounds
with (^) exocyclic double^ bonds,^ e.g.,^ 18.
- (^) Chirality due^ to^ a^ helical^ shape.^
Several compounds^ have^ been^ prepared^
that are
Chiral because^ they^ have^
a shape that^ is^ actually^
helical and^ can^ therefore^ be^ left-^ or
ght-handed in^ orientation.^
The entire^ molecule^ is^ usually^
less than^ one^ full^ turn^ of^ the
elx, but^ this^ does^ not^
alter the^ possibility^
of left-^ and^ right-handedness.^
An example is
exahelicene 53 in^ which^ one^
side of^ the^ molecule^
must lie^ above^ the^ other^
because of
a Press: New
CHAPTER 4 OPTICAL^ ACTIVITY^ AND^ CHIRALITY^109
The Cahn-Ingold-Prelog System
The system that^ has^ replaced^ the^ DL^ system^ is^ the^ Cahn-Ingold-Prelog^ system,^
in which the four (^) groups on^ an^ asymmetric^ carbon^ are^ ranked^ according^ to^ a^ set^
of (^) sequence rules.74^ For
our purposes we^ confine^ ourselves^ to^ only^ a^ few^ of^ these^ rules,^
which are sufficient^ to^ deal with the vast majority of chiral compounds.
- Substituents^ are^ listed^ in^ order^ of^ decreasing^ atomic^ number^
of the atom^ directly
joined to^ the^ carbon.
- Where^ two^ or^ more^ of^ the^ atoms^ connected^ to^ the^ asymmetric^
carbon are^ the^ same, the atomic^ number^ of^ the^ second^ atom^ determines^ the^ order.^ For^ example,^
in the^ molecule MeCH-CHBr-CH,OH, the^ CH2OH^ group^ takes^ precedence^
over the^ Me2CH group
because oxygen has^ a^ higher atomic^ number^ than^ carbon.^ Note^ that^
this is so^ even^ though
there are two^ carbons^ in^ Me,CH^ and^ only^ one^ oxygen^ in^ CH,OH.^
If two^ or^ more^ atoms connected to the^ second^ atom^ are^ the^ same,^ the^ third^ atom^
determines the precedence, etc.
3. All atoms^ except hydrogen are^ formally^ given^ a^ valence^ of^
- Where^ the^ actual^ valence
is less (as in^ nitrogen, oxygen, or^ a^ carbanion),^ phantom^ atoms^ (designated^ by^
a (^) subscript
o) are^ used^ to^ bring^ the^ valence^ up^ to^ four.^
These (^) phantom atoms^ are^ assigned^ an^ atomic
number (^) of zero and^ necessarily^ rank^ lowest.^ Thus^ the^ ligand-NHMe^
ranks (^) higher than
-NMe
- A tritium^ atom^ takes^ precedence^ over^ deuterium,^ which^
in turn takes^ precedence^ over
ordinary hydrogen.^ Similarly,^ any^ higher^ isotope^ (such^
as C) takes^ precedence^ over^ any
lower one.
5. Double^ and^ triple bonds^ are^ counted^ as^ if^ they^
were (^) split into^ two^ or^ three^ single
bonds, respectively, as^ in^ the^ examples^ in^ Table^
4.1 (^) (note the^ treatment^ of the^ phenyl^ group).
TABLE 4.1 How^ four^ common^ groups^ are^ treated^
in the Cahn-Ingold-Prelog system Group Treated^ as^ if^ it^ were Group
Treated as if it were
C
= -OC^ -CH=CH, H
Coe
-C=CH -C^ H-C- H
-cHs
"For descriptions^ of^ the^ system^ and^
sets of^ sequence rules,^ see^ Ref.^ 2;^ Cahn;^ Ingold;^ Prelog^ Angew.^ e Ed. (^) Eng. 1966, 5,^ 385-415^ [Angew.^ Chem.^ 78,^ 413-447];^
Cahn J. Chem.^ Educ.^ 1964,^ 41,^ 116;^ Fernelius,OCn Adams. Chem.^ Educ.^ 1974,^ 51,^ 735.^
See also (^) Prelog and^ Helmchen^ Angew.^ Chem.,^ Int.^ Ed.^ Engl.^
1982, 21,^ 30/*S
[Angew. Chem.^ 94, 614-631].
even (^) though it (^) has (^) two chiral (^) carbons. Tartaric (^) acid is (^) a
Isomers of tartaric acid: a pair of typical case. There are only three
enantiomers (^) and (^) an (^) inactive (^) meso (^) form. For compounds ÇOOH COOH H-OH HO-H
HO-H H-OH COOH
ÇOOH H -OH
H-OH COOH COOH dl pair (^) meso the three (^) stereoisomers of (^) tartaric acid
that have two chiral atoms, meso forms are found
the (^) chiral (^) atoms are only^ where^ the^ four^ groups^ on^ one^ of the (^) same (^) as those (^) on the (^) other chiral (^) atom.
In most cases with more than two chiral centers, the number of isomers
from the can^ be^ calculated
formula 27, where n is the number of chiral centers,
actual (^) number is (^) less although^ in^ some^ cases^ the than (^) this, (^) owing to meso (^) forms.82 An (^) interesting case is (^) that (^) of 2,3,4-pentanetriol (^) (or any similar (^) molecule). (^) The (^) middle (^) carbon is (^) not (^) asymmetric when the 2- and (^) 4-carbons (^) are both R (^) (or both (^) S) but is asymmetric when^ one^ of^ them is R^ and the
CH, SH-OH H-OH
ÇH
S H-OH
CH
SH-OH
CH,
RHO H
HO- H HO H H OH
RH-OH R H-OH CH,
s HO H
CH
R H-OH
H CH
meso meso (^) dl pair
other S. Such a (^) carbon (^) is called a (^) pseudoasymmetric carbon. (^) In these cases (^) there are four isomers: two^ meso^ forms^ and one dl (^) pair. The (^) student should (^) satisfy himself (^) or (^) herself, remembering the rules^ governing the^ use^ of^ the Fischer (^) projections, that these (^) isomers (^) are different, that^ the^ meso (^) forms are (^) superimposable on (^) their (^) mirror (^) images, and that there are no other (^) stereoisomers. Two (^) diastereomers (^) that have a (^) different (^) configuration at one chiral center^ are called only epimers. In (^) compounds with two or (^) more chiral (^) centers, the (^) absolute (^) configuration must (^) be sep-
We discuss asymmetlc
- Active^ substrate.^ Ifa^ new^ chiral^ center^ is^
created in a^ molecule^ that^ is^ already^ opticallyV
active, the^ two^ diastereomers^ are^ not^ (except^ fortuitously)^
formed in (^) equal amounts.^ The
reason is that^ the^ direction^ of^ attack^ by^ the^ reagent^ is^
determined (^) by the^ groups already
there. For^ certain^ additions^ to^ the^ carbon-oxygen^ double^ bond^
of ketones^ containing an
Me CN
Et-C-C-H
Me H^ OH
32 Et-C-C-H+ HCN
H Ö Me OH
Et-C-C-H
IL.
H CN
CHAPTER 4 OPTICAL^ ACTIVITY^ AND^ CHIRALITY^117
asymmetric a^ carbon,^ Cram's^ rule^ predicts^ which^ diastereomer^ will^ predominate.^
If (^) the molecule is^ observed^ along its^ axis,^ it^ may^ be^ represented^ as^ in^34 (see^ p.^ 139),^ where^ S, M, and^ L^ stand^ for^ small,^ medium,^ and^ large,^ respectively.^ The^ oxygen^ of^ the^ carbonyl
M YZ (^) M MI
ZO oz
34 Major product Minor product
orients itself^ between^ the^ smali-^ and^ the^ medium-sized^ groups.^ The^
rule is that the incoming
group preferentially^ attacks^ on^ the^ side^ of^
the (^) plane containing the^ small^ group.^ By^ this rule, it^ can^ be^ predicted^ that^33 will^ be^ formed^ in^ larger^
amounts than^ 32.
Many reactions^ of^ this^ type^ are^ known,^ in^
some of^ which^ the^ extent^ of^ favoritism^ ap- proaches 100%^ (for an^ example^ see^ 2-11).0^ The^ farther^ away^
the reaction^ site^ is^ from^ the chiral (^) center, the less^ influence^ the^ latter^ has^ and^ the^ more^ equal^ the^
amounts of^ diaster
eomers formed. In a (^) special case of this^ type of^ asymmetric^ synthesis,^ a^ compound^ (35)^
with achiral
molecules, but^ whose^ crystals^ are^ chiral,^ was^ converted^ by^
ultraviolet (^) light t a^ single enantiomer of a chiral (^) product (36).
Ifthere is more than one double bond!59 in a molecule and if W X and Y Z for each, the^ number of (^) isomers (^) in the most (^) general case is (^) 2", (^) although this number (^) may be decreased if some (^) of (^) the (^) substituents are the (^) same, as in
H (^) H CH H CH,
c=C CH (^) CH CH (^) CH CH H H (^) CH, H
C= H H H (^) CH, H cis-cis or cis-trans or trans-trans or Z, Z Z, E E, E
When a molecule contains a double bond and an asymmetric carbon, there are four isomers
a cis pair of enantiomers and a trans pair
H (^) CH, CH H^ H (^) ,CH, C,H H
CH3 CH H H
CH CH^ CH^ C=C
H
CH (^) C=C H H (^) CH H^ H CH Z or cis dl (^) pair E^ or^ trans^ dl^ pair
Double bonds^ in^ small^ rings are^ so^ constrained^ that^ they^ must^ be^ cis.^ From^ cyclopropene (a known^ system)^ to^ cycloheptene,^ double^ bonds^ in^ a^ stable^ ring^
cannot be^ trans.^ However,
the (^) cyclooctene ring is^ large enough to^ permit^ trans^ double^ bonds^ to^ exist^ (see^ p.^ 104),^
and
for (^) rings larger than^ 10-or^ 11-membered,^ trans^ isomers^ are^ more^ stable'"^ (see^
also (^) p. 158)
In a few^ cases, single-bond rotation^ is^ so^ slowed^ that^ cis^ and^
trans isomers^ can^ be^ iso 161 1 162) One example 1s
TABLE 4.2 (^) Some (^) properties of (^) maleic (^) and (^) fumaric acids
H COOH
C=o
HOOC
H H
C=C
HOOC coOH^ H
Property Maleic^ acid^ Fumaric^ acid Melting point, °C^286 Solubility in water at 25°C, g/liter K, (at 25°C) K2 (at 25°C)
130 788 7 1 x 10- 3x 10-s
1.5x 10- 2.6 x 10-
Since (^) they generally have^ more^ symmetry than^ cis^ isomers,^ trans^ isomers^ in^ most^ cases^ have
higher melting^ points^ and^ lower^ solubilities^ in^
inert solvents.^ The^ cis^ isomer^ usually has a
higher heat^ of^ combustion,^ which^
indicates a^ lower^ thermochemical^ stability.^ Other^ notice-
ably different^ properties^ are^ densities,^
acid strengths, boiling^ points,^ and^ various^ types^ of
spectra, but^ the^ differences^ are^ too^
involved to^ be^ discussed^ here.
Other notice-
one ring is^ four-membered is a four-five
known. For^ the^ junction;^ trans-bicyclo|3.2.0Jheptane^ (61)^ is bicyclo[2.2.0] (^) system (a four-four (^) fusion), (^) only cis (^) compounds have (^) been
H
H cis-Decalin (^) trans-Decalin^61
made. (^) The (^) smallest (^) known (^) trans junction when^ one^ ring is^ three-membered is a (^) six-three junction (^) (a (^) bicyclo|4.1.0] (^) system). An (^) example is (^) 62.68 When (^) one and (^) the (^) other ring^ is^ three-membered eight-membered (^) (an (^) eight-three (^) junction), the (^) trans-fused (^) isomer is (^) more
stable than the corresponding cis-fused
In isomer. bridged (^) bicyclic (^) ring systems, two (^) rings share more (^) than (^) two (^) atoms. In (^) these (^) cases there (^) may be (^) fewer (^) than 2" (^) isomers because of the (^) structure of the there (^) are (^) only two (^) isomers of system.^ For^ example, camphor (^) (a pair of^ enantiomers), (^) although it (^) has (^) two (^) chiral
Me Me H
Me camphor carbons. In (^) both (^) isomers the (^) methyl and is hydrogen^ are^ cis.^ The^ trans^ pair of^ enantiomers impossible in^ this^ case, since the (^) bridge must (^) be (^) cis. (^) The (^) smallest prepared in^ which^ the (^) bridge is (^) trans (^) is (^) the bridged^ system^ so^ far [4.3.1] (^) system; the^ trans (^) ketone (^63) has (^) been
=o
63
prepared.170 In^ this^ case^ there^ are^ four (^) isomers, since (^) both (^) the (^) trans also been (^) prepared) are^ pairs of and^ the^ cis^ (which has enantiomers. When one^ of the^ bridges contains a (^) substituent, the the isomers involved. When^ the^ two^ question^ arises^ as^ to^ how^ to^ name bridges that^ do^ not^ contain (^) the (^) substituent (^) are of
length, the^ rule^ generally followed^ is^ that the unequal
prefix endo-^ is^ used (^) when the (^) subst1tuent is
167Meinwald: Tufariello; Hurst J. Org. Chem. 1964, 29, 2914,
CHAPTER 4 CIS-TRANS^ ISOMERISM^133
closer to^ the^ longer of^ the^ two^ unsubstituted^ bridges;^ the^ prefix^ exo-^ is^ used^ when the
substituent is closer to the shorter bridge; e.g.,
OH H
H OH exo-2-Norborneol endo-2-Norborneol
When the^ two^ bridges^ not^ containing^
the substituent^ are^ of^ equal^ length,^
this convention
cannot be^ applied,^ but^ in^ some^
cases a decision^ can^ still^ be^ made;^ e.g.,^
if one^ of^ the^ two
bridges contains^ a^
functional group, the^ endo^
isomer is the^ one^ in^ which^ the^
substituent is
closer to^ the^ functional^ group:
H- CH^ CH^ -H
endo-7-Methyl-2- norcamphor
exo-7-Methyl-2- norcamphor
nolts of tricyclic
