Photoinduced Carboborative Ring Contraction Enables Regio- and Stereoselective Synthesis of Multiply Substituted Five-Membered Carbocycles and Heterocycles

Article abstract of DOI:10.1021/jacs.7b07128

We report herein a photoinduced carboborative ring contraction of monounsaturated six-membered carbocycles and heterocycles. The reaction produces substituted five-membered ring systems stereoselectively and on preparative scales. The products feature multiple stereocenters, including contiguous quaternary carbons. We show that the reaction can serve as a synthetic platform for ring system alteration of natural products. The reaction can also be used in natural product synthesis. A concise total synthesis of artalbic acid has been enabled by a sequence of photoinduced carboborative ring contraction, Rauhut-Currier reaction, and nitrilase-catalyzed hydrolysis. The synthetic utility of the reaction has been further demonstrated by converting the intermediate organoboranes to alcohols, amines, and alkenes.

Full text of DOI:10.1021/jacs.7b07128

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Communication
Photoinduced Carboborative Ring Contraction Enables
Regio- and Stereoselective Synthesis of Multiply
Substituted Five-Membered Carbocycles and Heterocycles
Shengfei Jin, Vu T. Nguyen, Hang T. Dang, Dat P. Nguyen, Hadi D. Arman, and Oleg V. Larionov
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Aug 2017
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Photoinduced Carboborative Ring Contraction Enables Re-
gio- and Stereoselective Synthesis of Multiply Substituted
Five-Membered Carbocycles and Heterocycles
Shengfei Jin,‡ Vu T. Nguyen,‡ Hang T. Dang, Dat P. Nguyen, Hadi D. Arman, and Oleg V. Larionov*
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Department of Chemistry, The University of Texas at San Antonio, San Antonio, Texas 78249, United States
Supporting Information Placeholder
under mild conditions in the absence of additives and catalysts. In
contrast to most other ring contraction processes, the photoin-
duced carboborative ring contraction results in an appendage of
an additional side chain with a new stereocenter. The structure of
the products 3 can be further diversified via conversion of the
boryl group in the side chain to other functional groups. A num-
ber of secondary metabolites were readily converted to function-
alized cyclopentanes with two new stereocenters, including qua-
ternary all-carbon stereocenters.
ABSTRACT: We report herein a photoinduced carboborative
ring contraction of monounsaturated six-membered carbocycles
and heterocycles. The reaction produces substituted five-
membered ring systems stereoselectively and on preparative
scales. The products feature multiple stereocenters, including
contiguous quaternary carbons. We show that the reaction can
serve as a synthetic platform for ring system alteration of natural
products. The reaction can also be used in natural product synthe-
sis. A concise total synthesis of artalbic acid has been enabled by
a sequence of photoinduced carboborative ring contraction, Rau-
hut-Currier reaction, and nitrilase-catalyzed hydrolysis. The syn-
thetic utility of the reaction has been further demonstrated by
converting the intermediate organoboranes to alcohols, amines
and alkenes.
Scheme 1. Photoinduced Carboborative Ring Contraction
Ring contraction reactions are among the most useful strategic
transformations for construction of complex carbocyclic and het-
erocyclic molecules. Favorskii, Wagner-Meerwein, pinacol and
Wolff rearrangements, as well as ring contraction reactions medi-
ated by hypervalent iodine and selenium reagents allow for effi-
cient construction of densely substituted and less accessible
smaller cyclic systems from the more abundant larger ones with
predictable and high stereoselectivity.1
Six-membered ring is widely represented among secondary me-
tabolites, e.g., terpenes and alkaloids. Cyclohexene motif can also
be easily accessed by a number of regio- and enantioselective
synthetic methods, e.g., the Diels-Alder cycloaddition.2 The
abundance and synthetic accessibility of the unsaturated six-
membered ring system makes it an excellent precursor to the less
readily accessible five-membered carbocycles and heterocycles.3
In addition, structural alteration, including ring system modifica-
tion, plays an important role in medicinal chemistry of natural
products, since it can lead to improved activity, metabolic stabil-
ity, and target specificity.4
Although earlier observations of the photoinduced reaction be-
tween cyclohexenes and trialkylboranes suggested that syn-
carboboration with retention of the cyclohexane ring took place,10
we observed a clean and efficient carboborative ring contraction
(see additional discussion in SI) that, after oxidation, resulted in
isolation of alcohol 4 as the sole product from the UV-induced (
= 254 nm) reaction of 1-methylcyclohexene and triethylborane
(Table 1). Further investigation showed that xylene isomers were
superior to other aromatic hydrocarbons as photosensitizers, with
p-xylene delivering a higher yield of alcohol 4 than m-, and o-
xylenes (91% for p-xylene, 82% for o-xylene, and 71% for m-
xylene). The reaction proceeded faster and with higher yields in
more polar solvents, e.g., in alcohols, with ethanol as the optimal
solvent. Other suitable solvents included dioxane, and tetrahydro-
furan. The photochemical quantum yield for the formation of
alcohol 4 was 0.26. The organoborane intermediate corresponding
to product 4 was observed by means of NMR spectroscopy, indi-
cating that the carboborative ring contraction is a photoinduced
process that takes place before the oxidative work-up.
Photochemical activation enables generation of chemical inter-
mediates that are not accessible from the ground states, e.g., E-
cyclohexenes,5 and triplet aryl cations,6 whose reactivity is dis-
tinctively different from the thermally-generated species.
Cyclohexenes undergo photosensitized isomerization to strained
and highly reactive E-isomers that readily participate in a variety
of reactions, e.g., additions of alcohols and cycloadditions.7 Ex-
perimental and computational evidence indicates that the more
stable chair conformation is responsible for the reactivity ob-
served for E-cyclohexene.8,9 Despite the high angular strain with-
in the ring, the reactions of E-cyclohexenes E-1 can be remarka-
bly stereoselective.7,8
Interestingly, the reaction can also be carried out with a catalytic
photosensitizer. Among photosensitizers evaluated,11 ethyl benzo-
We report herein an efficient photoinduced carboborative ring
contraction that enables a regioselective synthesis of multiply
substituted cyclopentanes 2 (Scheme 1). The reaction proceeds
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ate (20 mol%) was found superior, delivering product 4 in 95%
formed with very high stereoselectivity, as, in each case, the same
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yield at 254 nm (in tetrahydrofuran) and 300 nm (in ethanol).
configuration was observed for this stereocenter. Carvone-derived
tertiary alcohol 25 gave rise to product 26 with two adjacent qua-
ternary stereocenters in the newly-formed stereochemical triad
indicating that carboborative ring contraction can be used for
construction of molecules with contiguous quaternary stereocen-
ters.14 In addition, nerol oxide and valencene were readily con-
verted to tetrahydrofuran 28 and 6/5-fused bicyclic alcohol 30 in
52 and 74% yields.
We next examined the scope of the reactants (Table 1). A number
of boranes bearing primary and secondary alkyl groups were re-
acted with 1-methylcyclohexene under the optimal conditions.
Table 1. Scope of the Photoinduced Carboborative Ring
Contractiona
Table 2. Scope of the Photoinduced Carboborative Ring
Contraction of Terpenoidsa
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a
Reaction conditions: cycloalkene (0.5–1 mmol), trialkylborane
(1–1.5 mmol), EtOH (5 mL), p-xylene (2 mL), UV (254 nm), then
H2O2, NaOH, or Na2CO3·1.5H2O2. b 10 : 1 d.r.
a
Trialkylboranes were readily prepared by hydroboration of al-
kenes with borane-tetrahydrofuran complex.12 Alcohols 4-11 were
obtained in good yields, including product 8 derived from tricy-
clopentylborane, and trialkylboranes that contain an aromatic
group (10, 11). Interestingly, 9-methoxy-9-borabicyclo[3.3.1]-
nonane (9-BBN-OMe) also proved to be a suitable reacting part-
ner, and the diol 12 was isolated after oxidation in 63% yield.
Reaction conditions: terpenoid (1 mmol), trialkylborane (1–1.5
mmol), EtOH (5 mL), p-xylene (2 mL), UV (254 nm), then H2O2,
NaOH, or Na2CO3·1.5H2O2. b Yield of pure diastereomer 21 after
recrystallization. The minor diastereomer has the opposite config-
uration of the CH(OH) stereocenter in the side chain.
Steroids are functionally important biological molecules that play
key roles in signal transduction and cell membrane function.
Many steroids have found use in medicine and have been crucial
in the development of drugs and in the elucidation of fundamental
cellular processes.13,15 Norsteroids are an important subclass of
steroids.16 We, therefore, proceeded to test the photoinduced car-
boborative ring contraction reaction in the synthesis of B-ring
norsteroids. We found that a variety of naturally-occurring ster-
oids and steroid derivatives were converted to the corresponding
B-ring contraction products 31-45 (Table 3). The reactions pro-
ceeded with high regio- and stereoselectivity, presumably due to
the rigidity of the steroidal structure, and the products were isolat-
ed as single diastereomers. Cholesterol and diosgenin produced
the B-ring contraction products 31-36 with several trialkyl-
boranes. B-Norsteroids from derivatives of pregnenolone (37-39),
dehydroepiandrosterone (40-42), and azasteroids (43-45) were
also prepared. Single crystal X-ray crystallographic analysis of
the cholesterol-derived product 31 and the trifluoromethylated
triol 42 confirmed the stereochemical assignment of the newly-
formed stereocenters.
Other unsaturated six-membered carbocycles and heterocycles
were studied as well. Cyclohexene produced the corresponding
alcohols 13 and 14 in 98 and 90% yields, indicating that cyclo-
hexenes with the unsubstituted C=C bond can also be used in the
photoinduced carboborative rearrangement reaction. 1-tert-
Butylcyclohexene also produced the sterically hindered alcohol
15 in 51% yield. Experiments with unsaturated six-membered
oxygen and nitrogen heterocycles afforded tetrahydrofuran 16 and
pyrrolidine 17. While tetrahydrofuran 16 was isolated as a single
diastereomer, the diastereomeric ratio was 10:1 for pyrrolidine 17.
Terpenoids have important applications in medicine, agriculture,
organic synthesis, as well as flavor and fragrance industries.13 We
were, therefore, interested in examining the generality of the pho-
toinduced carboborative ring contraction reaction with several
readily available terpenoids and their derivatives (Table 2). Ter-
pinolene (18) produced the five-membered ring product 19 with
>20:1 stereoselectivity. (R,R)-Carveol (20) afforded product 21 in
a high yield and with 7–10 : 1 stereoselectivity. The minor dia-
stereomer had the opposite configuration at the carbon atom of
the alcohol stereocenter in the side chain. Alcohol 21 can be fur-
ther purified by recrystallization to >20:1 d.r. The stereochemical
assignment of the carveol product 21 was confirmed by X-ray
crystallography. Similarly, TBS ether of carveol 22 afforded al-
cohols 23 and 24 in 63 and 58% yields, respectively. The synthe-
ses of products 21 and 23 were readily carried out on gram scales.
Interestingly, the all-carbon quaternary stereocenter in 21-24 was
1-Methylcyclohexene reacted 1.9 times faster than cyclohexene.
This result, in addition to the higher reaction rates in more polar
solvents6 may indicate that polar intermediates are involved in the
photoinduced carboborative ring contraction process. Existing
experimental evidence shows that the protonation of the C=C
bond in E-cyclohexenes occurs stereoselectively from the outside
face of the E-cyclohexene ring leading to an equatorial C–H bond
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in the resulting cyclohexyl cation.8 The trialkylborane addition
fore, developed for this conversion. In the first one, nitrile 49 was
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from the outside face of E-cyclohexene will result in dipolar in-
termediate I (Scheme 1).
hydrated to the primary amide with the aid of the Parkins catalyst
50.21 The primary amide was then converted to the corresponding
N,N-bis-Boc-imide,22 that was hydrolyzed to the acid at ambient
temperature. Cleavage of the TBS group afforded artalbic acid
(46). Alternatively, conversion of nitrile 49 to artalbic acid (46)
was accomplished in two steps. In this route, desilylation of nitrile
49 was followed by a biocatalytic hydrolysis of the nitrile group
to the carboxylic acid that was effected by nitrilase23 at pH 7.2.
Table 3. Scope of the Photoinduced Carboborative Ring
Contraction of Steroid Compoundsa
Scheme 2. Synthesis of Artalbic Acid
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The C–B bond in organoboranes can be readily converted to a
variety of functional groups.12,24 For example, the photoinduced
carboborative ring contraction of carveol 20 and its TBS ether 22
was followed by a reaction with 2-methyl-2-nitrosopropane di-
mer25 resulting in the formation of E-alkenes 51 and 52 that were
isolated as single isomers in 68 and 70% yields (Scheme 3). Cho-
lesterol-derived B-norsteroid 53 with an E-alkenyl side chain was
also prepared using the same procedure. Further, 4-alkoxyphenol
54 was readily prepared by a photoinduced carboborative ring
contraction of 1-methylcyclohexene, followed by treatment with
benzoquinone.
a
Reaction conditions: steroid derivative (0.23–0.5 mmol), trial-
kylborane (0.3–0.6 mmol), EtOH or THF (2–4 mL), p-xylene
(0.5–1 mL), UV (254 nm), then H2O2, NaOH.
The following migration of the endocyclic C3 atom to C1 can be
accompanied by a migration of one of the alkyl groups R from
boron to C2 position. Our experimental data indicate that the mi-
gration of the C3 atom to C1 results in the trans-configuration of
the borylalkyl group (C2) with respect to the equatorially oriented
substituent at C4 in the rearrangement product 3. A similar ring
contraction step was proposed to explain the stereoselection in the
terminal step of the Prins-pinacol rearrangement of allylic diol-
derived acetals.17 The migration of the alkyl group R from boron
to C2 position can proceed with inversion or retention of the con-
figuration at C2. Experimentally, the observed configuration at
C2 in the major product 3 corresponds to the inversion pathway,
while the minor product 3' can be formed by the retention path-
way.
Scheme 3. Structural Diversification of the Carboborative
Ring Contraction Products
Photoinduced carboborative ring contraction also enabled a con-
cise total synthesis of artalbic acid (46)18 (Scheme 2). The pho-
toinduced reaction of (S,S)-carveol-derived TBS ether 47 with
triethylborane was followed by conversion to unsaturated ketone
48 by a sequence of oxidations with trimethylamine N-oxide and
Dess-Martin periodinane, -selenylation, and hydrogen peroxide-
induced selenoxide elimination.19 2-Cyanoethyl group was then
appended to the -position in enone 48 by means of a phosphine-
catalyzed Rauhut-Currier reaction20 with acrylonitrile as the only
acrylic acid derivative that afforded the cross-Rauhut-Currier
product. Nitrile 49 decomposed in strongly acidic and basic solu-
tions at the higher temperatures that are typically required for the
hydrolysis of nitriles to carboxylic acids. Two routes were, there-
Interestingly, although formation of 2-alkylhydroquinones had
previously been reported for a reaction of trialkylboranes with
benzoquinone,26 O-alkylation product 54 was isolated as a major
product in this case, in line with the reactivity pattern previously
observed
for
sterically
hindered
secondary
B-
alkylcatecholboranes.27 In addition, amination28 of the 1-
methylcyclohexene-derived organoborane intermediate with hy-
droxylamine-O-sulfonic acid afforded amine 55.
In conclusion, this paper describes a regio- and stereoselective
photoinduced carboborative ring contraction. The operationally
3
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simple reaction produces substituted five-membered carbocycles
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3 can further serve as precursors to alcohols, amines, and E-
alkenes.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
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Experimental procedures; characterization data (PDF)
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Crystallographic data for 31 (CIF)
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AUTHOR INFORMATION
Corresponding Author
oleg.larionov@utsa.edu
Author Contributions
‡These authors contributed equally
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support by the Welch Foundation (AX-1788), the NSF
(CHE-1455061), NIGMS (SC3GM105579), and UTSA is grate-
fully acknowledged.
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