Treatment of an internal acetylene such as 1-silyl-1-octyne (3) with a low-valent titanium reagent, (η 2 -propene)Ti(O-i-Pr) 2 (1) readily prepared from Ti(O-i-Pr) 4 and i-PrMgCl in situ, generates an acetylenetitanium complex. This complex was allowed to react with a terminal acetylene, 4-(benzyloxy)-1-butyne (5), to give the intermediate titanacyclopentadiene (6) which, upon hydrolysis, deuteriolysis, or iodinolysis, gave diene 8, or its bis-deuterated (>99% d 2 ) and bis-iodinated counterparts (9 and 10), in good yields and with high selectivities. This reaction is applicable to the cross-coupling reaction of functionalized acetylenes such as 2-alkynoates and 2-alkynamides 12-18 and a variety of terminal acetylenes 24-28 to give dienes 36-50 in good yields. A terminal acetylene 28 carrying an olefinic bond at the other terminus reacted with a silylpropiolate to afford the expected diene 42 without any complication.
Hydrometalation of acetylenes illustrated in eq 1 is a superior method for preparing stereodefined alkenylmetal species, which are particularly useful for the construction of olefinic linkages with predictable stereochemistry. Hydrometalation reactions1 including hydroboration,18 -alumination,lb -magnesation,lc -silylation,ld -stannation,18 and -zirconation,lf etc., have been routinely utilized in the synthesis of complex natural products. Herein we report a new entry to this series of reactions, namely the first hydrozincation of acetylenes, and demonstrate its application to further synthetic transformations.Recently, organozinc compounds of the type RZnX (R = alkyl; X = halogen, R, etc.) have been widely recognized as versatile carbon nucleophiles in organic synthesis owing to their selective reactivities.2 Alkenylzinc compounds are usually prepared via transmetalation of appropriate organometallic reagents, typically organolithium, -magnesium, or -boron compounds, with zinc salts or diethylzinc.2a-C|3 Direct reductive metalation of alkenyl halides with zinc metal can also be effected, but the stereochemical integrity of the starting alkenyl halide is not always retained in the zinc reagent.4 If hydrozincation of acetylenes (eq 1, "M-H" = "ZnHX") could be executed in a stereoselective manner, this reaction could become a straightforward method to prepare stereochemically defined alkenylzinc reagents. While hy-
tert-Butyl 2-en-7-ynoate 6 was
treated with
(η2-propene)Ti(O-i-Pr)2
(3), generated in
situ from
Ti(O-i-Pr)4 or Ti(O-i-Pr)3Cl and
i-PrMgCl, in ether at −50 to −20 °C to afford the
product 8 in good yield. The presence
of the intermediate titanabicycle 7 was verified by
bis-deuterolysis with excess D2O. When the
titanabicycle 7 was
treated with 1.1 equiv of i-PrOD and then worked up as
usual, the monodeuterated product 10 was obtained
with
high site selectivity and stereoselectivity. Other electrophiles
such as aldehydes and ketones also reacted with the
titanabicycle in a highly stereoselective manner to give cyclopentanes
having a stereo-defined side chain. On the
contrary, treatment of the corresponding ethyl ester, ethyl
8-(trimethylsilyl)-(E)-2-octen-7-ynoate (28),
with 3 under
the same conditions followed by the addition of 1.1 equiv of
s-BuOH afforded
2-(trimethylsilyl)-1-bicyclo[3.3.0]octen-3-one (32) in 80% yield. Quenching the same
reaction mixture with i-PrOD, EtCHO, and
Et2CO in place of
s-BuOH gave 4-deuterio (with exclusive deuterium
incorporation), 4-(1-hydroxypropyl), and
4-(1-ethyl-1-hydroxypropyl) derivatives of the above bicyclic ketone (34,
35, and 36) in good yields. These
electrophiles were always
introduced from the convex face of the bicyclic skeleton. The
stereochemistry of the cyclization could be controlled
by an allylic substituent such as
(tert-butyl)dimethylsiloxy or butyl group to a high
degree yet with a reversal
diastereoselection to give 45 or 47. The
reaction of ethyl 7-octen-2-ynoate (56) and 3 at
−50 to 0 °C took place in
a quite different way to afford
1-[(ethoxycarbonyl)methyl]bicyclo[3.1.0]hexane
(64) in 74% yield after hydrolysis.
If the simple hydrolysis is replaced by deuterolysis or the action
of diethyl ketone,
1-[(ethoxycarbonyl)dideuteriomethyl]
(with 99% deuterium incorporation), or
1-[(ethoxycarbonyl)(3-pentylidene)methyl] derivative of the
above product
(65 or 66) was obtained in good yields. A
7-en-2-ynoate having an internal Z-double bond such as
80 afforded a
single stereoisomer 82 with the substituent at the
endo position of the bicyclic skeleton, suggesting that
the
stereochemical integrity of the Z-double bond of the
starting material was retained in the product. An alkyl
substituent
at the allylic position of the substrates like 74 and
76 nicely controlled the stereochemistry of the cyclization
to
afford single products 75 and 77 with the
substituent being placed in the exo orientation of the
bicyclic structure.
This high diastereoselectivity was successfully applied to an
enantioselective synthesis of d-sabinene from an
optically
active enynoate via nearly complete chirality transfer.
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