.
Angewandte
Communications
Scheme 3. Deuterodeboronation of 18.
Supporting Information).[22] The spectra showed that supra-
facial incorporation of deuterium had occurred, which is
consistent with our proposed cyclic TS 20.[23]
In order to further demonstrate the utility of this method-
ology, a synthesis of a component (21,[24] Scheme 4) of the sex
Figure 2. Proposed transition states for transformations of a-substi-
tuted allylic boronates.
esters with aldehydes (Figure 2).[19] In these systems, low to
moderate Z-selectivity is observed with pinacol esters. The
outcome of such allylboration has been rationalized on the
basis of competing steric interactions. When the a-substitu-
ent, R, is in an equatorial position in the six-membered-chair
transition state (TS) 14 (leading to the E-isomer), it suffers
from severe gauche interactions with the bulky pinacol group
and minor A1,2 strain with the vinylic substituent (usually
a proton). In contrast, when the substituent is in an axial
position (as in 15, leading to the Z-isomer), it only suffers
from A1,3 strain. The Z-selectivity observed is indicative that
the gauche interactions with the bulky pinacol ester are much
greater than the A1,3 strain.
We propose that the Z-selective protodeboronation
proceeds through a similar six-membered TS, 16, whereby
fluoride both activates the boronic ester as the “ate” complex
and directs addition of water to the g-position. The enhanced
Z-selectivity observed in protodeboronation over allylbora-
tion of aldehydes is likely to be a consequence of the
enhanced A1,2 strain between the R group and the methyl
substituent in addition to the factors described above.
When the pinacol ester is exchanged for the less bulky
trifluoroborate salt (or, upon hydrolysis, the boronic acid) the
balance of steric forces is altered such that A1,3 strain now
constitutes the dominant factor. The reaction now proceeds
through TS 17, in which the a-substituent is in a pseudoequa-
torial position, leading to the E-product. The lower selectiv-
ities observed in this reaction can be attributed to the more
balanced steric demands of the system. The acid-catalyzed
reactions mirror the sense and degree of selectivity that can
be observed in TFA-mediated protonation of similarly
substituted allyl silanes.[20]
Scheme 4. Total synthesis of a component of the sex pheromone of
the Californian red scale insect. a) 1) O3, MeOH, À788C; 2) NaBH4,
MeOH, 08C, 82% over 2 steps; b) NaH, THF, 08C, then TIBCl, reflux,
48 h, 86%; c) TsCl, Et3N, DMAP, CH2Cl2, 93%; d) tBuOK, hexane, RT,
96% (regioselectivity=14:1); e) sBuLi, TMEDA, Et2O, À788C, 4 h,
then 27, À788C, 1 h, heating to reflux, 2 h; f) TBAF·3H2O, THF, RT,
2 h, then 458C, 16 h, 73% over 2 steps (d.r.>95:5); g) Ac2O, pyridine,
RT, 1.5 h, 99%; h) TBSCl, imidazole, DMAP, CH2Cl2, RT, 99%; i) CuCl
(5 mol%), PPh3 (6 mol%), tBuOK (20 mol%), B2pin2, MeOH, THF,
RT, 16 h, 88% (d.r. >95:5). DMAP=4-dimethylaminopyridine, TBAF·3
H2O=tetra-n-butylammonium fluoride trihydrate, TBSCl=tert-butyldi-
methylsilyl chloride, TIBCl=2,4,6-triisopropylbenzoyl chloride, TME-
DA=N,N,N’,N’-tetramethylethylene diamine.
pheromone of the Californian red scale beetle, Aonidiella
aurantii, was undertaken. The Californian red scale beetle is
a major citrus crop pest found in many areas of the world; and
as such, 21 is used in pest control[25] by acting as an attractant
to the mobile male members of the species. This compound
has been previously synthesized on several occasions,[26] and
often acts as a benchmark for the effectiveness of Z-selective
trisubstituted olefination methodologies. We believed that the
In order to determine whether protonation occurs supra-
or antarafacially, a diastereomerically pure allylic boronic
ester 18 was prepared.[21] Subsequent treatment with
TBAF·3D2O gave a single diastereomer of the deuterated
product [D1]-19 (Scheme 3), the stereochemistry of which was
determined by quantitative nOe distance analysis (see the
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12444 –12448