A-heteroatom-substituted

The diastereoselectivity is observed in the Henry reaction using optical active niti o compounds or a-heteroatom substituted aldehydes. Lor example, the reaction of O-benzyl-D-lactal-dehyde with methyl 3-niti opropionate in the presence of neubal alumina leads to a mixture of three niti o-aldol products from which D-ribo isomer is isolated by direct crystallization. D-Ribo... 

Several detailed studies of reactions of achiral aiiylboronates and chiral aldehydes have been reported4,52 - 57. Diastereofacial selectivity in the reactions of 2-(2-propenyl)- or 2-(2-butenyl-4,4,5,5-tetramethyl-l,3,2-dioxaborolanes with x-methyl branched chiral aldehydes are summarized in Table 252, 53, while results of reactions with a-heteroatom-substituted aldehydes are summarized in Table 34,52d 54- 57. 

The minimization of gauche pentane interactions in the transition stales is also an important consideration in the reactions of substituted allylboronates and a-heteroatom-substituted aldehydes4,52d 54,56. Transition states 8 and 11 have been identified by Roush and Hoffmann as the least sterically hindered ones accessible in reactions with (E)- and (Z)-allylboronates. 

The Andersen sulphoxide synthesis allows one also to synthesize a variety of a-heteroatom substituted sulphoxides starting from a-heteroatom stabilized carbanions and (—)-(S)-276. The selected examples shown in Scheme 3 are the best illustration of the generality of this approach. The reaction of enolates or enolate like species with (—)-(S)-276 has been used for the synthesis of optically active a-carbalkoxy sulphoxides. For example, treatment of (—)-(S)-276 with the halogenomagnesium enolates of -butyl acetate, t-butyl propionate or t-butyl butyrate resulted in the formation of ( + )-(R)-t-butyl p-toluenesulphinylcarboxylates 298367 (equation 163). 

Phenylthio)carbene,18 a heteroatom-substituted carbene, reacts similarly with a variety of alkoxides to give a-thiomethyl-substituted alcohols 14 (Scheme 8... 

Thus, metalation of cyclic nitrones, followed by successive reactions with electrophilic reagents serves as a synthetic method toward a-heteroatom substituted nitrones, which are inaccessible by other methods. It is noteworthy that these reactions can take place only with cyclic nitrones with E -configuration of the aldonitrone group. 

Only limited precedent exists for the stereoselective enolization and subsequent condensation of a-heteroatom-substituted esters 48a and 48b (eq. [29]). Ireland has examined the enolization process for a-amino ester derivatives where the Claisen rearrangement (chair-preferred transition states) was employed to ascertain enolate geometry (Scheme 10) (43). These results imply that 48a [X = N(CH2Ph)2 ] exhibits only modest selectivity for ( )-enoIate formation under the... 

Oxidative Decarboxylation of a-heteroatom-substituted Carboxylic Acids... 

As well as alkenylstannanes [106-108], other classes such as a-heteroatom-substituted alkyltributylstannanes [109] and, more importantly, allylic stannanes [110, 111] also undergo these Sn-Cu transmetalations. Otherwise difficult to prepare, allylic copper reagents may, however, be obtained by treatment of allylic stannanes (produced in turn from organolithium, magnesium, or zinc organometallics) with Me2CuLi LiCN. They enter into cross-coupling reactions with alkyl bromides [110] or vinyl triflates (Scheme 2.52) [111]. 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

Polysilanes (or polysilylenes) consist of a silicon-catenated backbone with two substituents on each silicon atom (Structure 1). The groups R and R attached to the silicon chain can be of a large variety. Polysilanes with alkyl and/or aryl substituents have been the most thoroughly investigated [1-3], whereas polysilanes having at least a heteroatom substitution such as H, Cl, OR, NR2 have received much less attention [4]. The number of silicon atoms is usually from several hundreds to several thousands. 

Aldehydes bearing a-hetero substituents also typically afford anti products, and the general solution to syn selective a-heteroatom substituted aldehyde-aldehyde aldol processes via enamine catalysis also still remains to be discovered. Nevertheless, the anti process is remarkably useful because a variety of highly substituted aldehydes can be accessed in a single operation using only very inexpensive catalysts, such as proline 6 or the phenylalanine-derived imidazohdinone 46 (Scheme 21) [114, 116, 117, 119-121, 188]. 

Generally, a-heteroatom-substituted alkyl and vinyl lithium compounds are generated in solution under inert conditions at appropriate low temperatures and allowed to react further without isolation. The acidity of monohalogen-substituted alkanes is usually not high enough to permit the generation of a-lithio-a-halogen alkanes by deprotonation. 

II. UNUSUAL CHARACTERISTICS OF A/-HETEROATOM-SUBSTITUTED HYDROXAMIC ESTERS... 

A-Heteroatom-substituted hydroxamic esters can be expected to have different properties to those displayed by simple hydroxamic esters in two respects. On the one hand, bisheteroatom substitution at nitrogen impacts strongly upon the nitrogen hybridization... 

A. Amide Properties in A/-Heteroatom-substituted Hydroxamic Esters... 

The barrier to inversion at nitrogen in A-heteroatom-substituted hydroxamic esters should be greater than that found for hydroxamic esters or simple amides. However, it is likely to be substantially reduced in anomeric amides relative to amines since the planar transition state in which nitrogen is sp hybridized, can benefit from jr-overlap with the carbonyl (Figure 3b) and this has been verified experimenfally Rudchenko has measured an inversion barrier for A,A-dialkoxyureas af AG = 9-11 kcalmoH and fhose of acyclic dialkoxyamines fypicaUy af AG = 20-22 kcalmoH ... 

Af,Af-Dialkoxyamides (Ic) are the least studied class of stable A-heteroatom-substituted hydroxamic esters. The combined electronegativity of two alkoxyl oxygen atoms is less than that of acyloxyl and alkoxyl oxygen in A-acyloxy compounds but properties are largely typical of anomeric amides. There is good spectroscopic evidence that the nitrogen... 

In some cases, they act as magnesium carbenoids however, they usually react as a-heterosubstituted Grignard reagents. Therefore, the author will not address the chemistry of these a-heteroatom-substituted Grignard reagents in detail. 

TABLE 4. Reactions of carbonyl compounds with a-heteroatom-substituted dizinc species 1... 

A functionalized amino acid derivative has been synthesized by alkylation of a heteroatom-substituted Jt-allyl intermediate (Scheme 8E.27) [146], The alkylation of a Schiif base, which forms a 2-aza-7t-allyl intermediate, furnishes the malonate adduct with 85% ee despite the epimerizable nature of the newly generated stereogenic center. 

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