Subject reaction with aldehydes

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

As shown in Scheme 3, they prepared two possible stereoisomers of 20 [21]. Introduction of a methyl group by Grignard reaction to +)- R)-pulegon (22) gave 23, which was subjected to ozonolysis to afford 24. The tertiary hydroxyl group was protected as TMS ether (25) and subsequent aldol reaction with aldehyde 7 provided (-)-(2R,4R,6R,aS)-STV-C2 (20a) as a single diastereomer. On the other hand, racemic ketone ( )-ll was hydroxylated via... 

Brigaud and Portella et al. applied Yb(OTf)3 to aldol reaction of a,a-difluoroenol silyl ether (2) affording difluoromethylene ketones, a common structural motif of HlV-1 protease inhibitor [4], (2) was generated from acylsilane and trifluo-romethyltrimethylsilane and directly subjected to the aldol reaction with aldehydes with 10 mol% ofYb(OTf)3 in a one-pot procedure (Scheme 13.1). The same reaction with other Lewis acids such as TiCU or BF3-OEt2 required more than stoichiometric amount. 

O-Alkyl a-(trimethylsilyl)thioacetates and S-alkyl a-(trimethylsilyl)dithioacetates 119 can be deprotonated and subjected to Peterson reactions with aldehydes to give the a,jS-unsaturated thiocarboxylic acid O-esters and dithiocarboxylic acid S-esters 120, respectively (Scheme 2.72). The alkenes are obtained -selectively [200]. 

When C,0,0-tris(trimethylsilyl)ketene acetals such as 14S are subjected to the reaction with aldehydes in the presence of a catalytic amount of a Lewis acid followed by hydrolysis, the a,/ -unsaturated carboxylic acids are formed in a highly E-selective manner (Scheme 2.93) [265, 266]. Fluoride ion catalysts such as NaF and CsF are also used instead of the Lewis acids. 

Cyclohepta-3,5-dienone)iron complexes can be stereoselectively methylated and hydroxylated. The electrophile adds exclusively anti to the tricarbonyliron fragment. Double methylation or hydroxylation of the a and a positions is accomplished in high overall yield (Scheme 4-146). Silyl enol ethers adjacent to tricarbonyl(Ti -diene)iron units can be subjected to Mukaiyama aldol reaction with aldehydes to provide aldol adducts with varying diastereoselectivity. This methodology has, for example, been applied to the enantioselective synthesis of the dienetriols streptenol C and D (Scheme 4-147). ... 

Et2Zn also participates in the reductive coupling as a formal hydride source. Results for the Ni-catalyzed, Et2Zn-promoted homoallylation of carbonyl compounds with isoprene are summarized in Table 7 [30]. Et2Zn is so reactive that for the reaction with reactive aromatic aldehydes it causes direct ethylation of aldehydes, and the yields of homoallylation are diminished (runs 1 and 2). Unsaturated aldehydes seem to be subject to the Michael addition of Et2Zn. Accordingly, for the reaction with cinnamaldehyde, none of the expected homoallylation product is produced instead, the 1,4-addition product of Et2Zn, 3-phenylpentanal is produced exclusively (run 3). 

Because in this case an elimination reaction could occur to form an a,/ -unsaturated ester, use of two equivalents of the phosphorane should be avoided.1The (a-acylalk-ylidene)triphenylphosphorane can be subjected to a subsequent Wittig reaction with an aldehyde.[4] [5],[8]... 

Reaction with a first aldehyde transforms 176 into the vinylphosphonium chloride 177, which for practical reasons is subjected to an anion-exchange process, leading to the phosphonium salt 178. From this, phenyllithium treatment liberates the allenic phosphorane 179, an intermediate that has previously been used to prepare allenes from aldehydes [69], in the present case providing the products 180. The same protocol has also been applied to o-alkynylbenzaldehydes to yield allenes of interest as model compounds for the study of Schmittel and Myers-type cyclization reactions [70]. 

The application of asymmetric synthesis through reaction of chiral enolates with aldehydes is commanding a great deal of current interest, and aspects of this topic have been the subject of a number of recent... 

Table 4.8 Porphyrin aldehydes subjected to the Prato reaction with sarcosin.

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