August 1998
SYNLETT
909
Solvent Assisted Addition of Tetraallylic, Tetraallenic and Tetrapropargylic Stannanes to
Aldehydes and Acetals
1
2
2
2
Adam McCluskey* , I Wayan Muderawan , Muntari and David J Young*
1
Department of Chemistry, The University of Newcastle, University Drive, Callaghan, Newcastle, NSW Australia 2308;
Email: amcclusk@mail.newcastle.edu.au
2
School of Science, Griffith University, Nathan, Qld, Australia, 4111;
Received 18 May 1998
Abstract: Tetraallylic, tetraallenic and tetrapropargylic stannanes (0.25
eq) react with aldehydes in methanol to provide unsaturated alcohols.
These reactions proceed exclusively with allylic rearrangement for
tetra(2-butenyl)tin 2b and tetra(1,2-butadienyl)tin 5e and predominantly
with allylic rearrangement for tetrapropadienyltin 5c and tetra(2-
butynyl)tin 6e. The corresponding TFA catalysed reactions of dimethyl
acetals with 5c and 6e are highly regioselective with allylic
Scheme 1
rearrangement.
The allylation of carbonyl groups provide homoallylic alcohols which
1
possess useful functionality for further elaboration. A large number of
protocols have been developed which allow this transformation to be
2
achieved with high levels of regio- and stereocontrol. The related
propargylation and allenylation of aldehydes has also received
considerable attention over the past decade, particularly by Marshall and
2a
coworkers. This group has developed a variety of methods to achieve
regio- and stereocontrol which they have employed for the asymmetric
3
synthesis of complex natural products. We recently reported a
particularly mild and convenient procedure for the chemoselective
allylation of aldehydes and some ketones with commercially available
4
tetraallyltin. The carbonyl compound and stannane (0.25 eq) react
methyl group at the terminal position yielded tetrapropargylic stannane
6e while the other propargylic halides not substituted at this position
provided tetraallenic stannanes 5c or 5e exclusively. Propargylic
triarylstannanes are reported to isomerise in methanol to the
quantitatively in methanol at room temperature (aldehydes) or at reflux
(ketones) over approximately 4 to 20 h. The resulting homoallyl alcohol
can be easily separated from insoluble tin methoxide salts. Unlike the
corresponding reactions of allyltrialkylstannanes, this procedure does
not require anhydrous conditions, the use of expensive catalysts or
10
corresponding allenyl isomer depending on the substitution pattern.
No isomerisation of 5c, 5e or 6e was observed in methanol after 48 h at
room temperature suggesting that the Grignard reaction yields the
thermodynamically favoured product in each case.
chromatography to remove organotin by-products. Acetals are also
5
allylated with this reagent, but require the addition of TFA or SiO .
2
This latter procedure is particularly suited to the reaction of unstable
amino aldehydes which are more conveniently handled as the
corresponding acetals.
We have previously suggested that the methanol promoted allylation of
carbonyl compounds might be concerted with the activating influence of
the solvent primarily a result of hydrogen bonding to the carbonyl
4b
oxygen. If this is the case, then allylation should be regiospecific with
addition of the aldehyde or ketone to the γ- position of the allylic triad.
We now report an investigation of the regiochemistry of this reaction
and extend it to the analogous propargylation and allenylation reactions.
Scheme 2
Tetraallylic stannanes 2a and 2b (0.25 eq) were reacted with aldehydes
in methanol at room temperature (ca 25 C) for 24 h (Scheme 3) and
Methyl substituted tetraallylic stannanes 2a and 2b were prepared from
o
the corresponding allylic chlorides 1 by a Grignard reaction (Scheme
provided the corresponding homoallylic alcohols cleanly in 60 - 83%
yield after distillation (Table 2). Aldehyde addition to stannane 2b was
highly regioselective proceeding with allylic rearrangement, but with
low diastereoselectivity in favour of the erytho isomer 9 over the threo
isomer 10 (d.e. = 0 - 30%).
6
1). Chloride 1b (E
: Z = 90 : 10) yielded a mixture of
tetraorganostannanes which isomerised to an almost perfect binomial
7
distribution of the five diastereomers of 2b on standing (Figure 1).
Stereochemical assignments were based on comparisons with the
13
119
8
corresponding C and Sn NMR data for 2-butenyltributylstannane.
The corresponding reactions of tetraallenic stannanes 5c and 5e and
tetrapropargylic stannane 6e with aldehydes 7 (Scheme 4) in methanol
(4 - 24 h, ca 30 C) also proceeded cleanly and in good yield (74 - 88%,
Table 2). The addition of aldehydes to tetrapropadienyltin 5c was
regioselective in favour of the allylically rearranged homopropargylic
alcohols 13, but contaminated with up to 30% of the isomeric allenyl
Tetrapropargylic and tetraallenic stannanes were prepared from the
corresponding propargylic chlorides 3 and bromides 4 by a Grignard
o
reaction in the presence of a catalytic amount of HgCl (ca 2 mol%)
2
9
(Scheme 2). Each tetraorganostannane was obtained as a single
regioisomer determined by the substitution pattern of starting
propargylic halide (Table 1). Thus, propargylic halides 3e and 4e with a
McCluskey, Adam
