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Chain extension of an aldehyde by two "propionate" units has been attained by stereoselective allylboration with the chiral 1-methylbutenyl boronate 3 to give, e.g., the homoallylic alcohol 6, followed by a regioselective hydroboratiod carbonylation procedure to give, e.g., the epimeric aldehydes 8. The latter were converted into the lactols 10, which equilibrated to the desired epimer. The lactols could again be subjected to an allylboration reaction, initiating a second round of the chain extension protocol. This technique has been used to synthesize in 15 steps the venturicidene C-15/C-27 segment 23, containing 8 stereogenic centers with the proper absolute configuration.The venturicidines 1 are antifungal antibioticsr2I whose structure and absolute configuration were established by Xray crystallography already in the 1960'~[~1. It was only recently that syntheses of the venturicidine aglycon were com~l e t e d [~,~] .The polypropionate-derived structural segment C-15/C-27 is remarkable, because the sequence of the stereogenic centers resembles that in syndiotactic polypropylene and might have particular conformational properties [6]. In fact, this segment of the molecule is fully extended in the crystalline stater3]. Moreover, this particular sequence of stereogenic centers is an attractive target to develop new and more efficient strategies for stereoselective synthesis. Thus, one strategy based on ring opening of an octalin nucleus has recently been reportedE71. venturicidineWe were interested to develop an iterative procedure for acyclic structures such as that found in the venturicidine C-15IC-27 segment, and we envisaged a sequence of two operations, by which an aldehyde could be extended by two propionate subunits. In the first operation an aldehyde 2 is converted to a homoallylic alcohol 4L81 under reagent control of diastereoselectivity. Use of the chiral 1 -methylbut-enyl boronate 3 should guarantee high levels of asymmetric i n d u c t i~n~~~~~~" ] . OH 0The second step consists of a stereoselective hydroformylation to convert the alkene 4 into the aldehyde 5. The latter should be the starting point for the next round of this chain extension protocol. The realization of this plan has been communicated before['*]. We report here on the details of its execution. Exploratory StudiesThe chain elongation of the alkene 4 to the aldehyde 5 is formally a hydroformylation reaction. Hydroformylations of homoallylic alcohols with a terminal alkene have been realized before[l3I, but little is known about the regio-and stereoselective hydroformylation of internal alkenes[141. For this reason the indirect hydroformylation sequence developed by Brown['5] seemed attractive to us. We therefore tested the regio-and stereoselectivity of this reaction on the model system 6[l61.Hydroboration of 6a with 9-BBN proceeded with full regioselectivity with formation of the alcohol 7a, while the diastereoselectivity remained modest. We therefore examined briefly the effect of other alcohol-protecting groups on the diastereosel...

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