Acyl equivalents

Sulfur compounds are useful as nucleophilic acyl equivalents. The most common reagents of this type are 1,3-dithianes, which on lithiation provide a nucleophilic acyl equivalent. In dithianes an umpolung is achieved on the basis of the carbanion-stabilizing ability of the sulfur substituents. The lithio derivative is a reactive nucleophile toward alkyl halides and carbonyl compounds. 11... 

Weik and Rademann have described the use of phosphoranes as polymer-bound acylation equivalents [65]. The authors chose a norstatine isostere as a synthetic target and employed classical polymer-bound triphenylphosphine in their studies (Scheme 7.54). Initial alkylation of the polymer-supported reagent was achieved with bromoacetonitrile under microwave irradiation. Simple treatment with triethyl-amine transformed the polymer-bound phosphonium salt into the corresponding stable phosphorane, which could be efficiently coupled with various protected amino acids. In this acylation step, the exclusion of water was crucial. 

This procedure is an excellent method to prepare 1,4-dicarbonyl compounds 163 (Scheme 7.48) and, using triethylamine, has been extended to include other activated double bonds.Thus, the starting a-amino acids can be considered as nucleophilic acyl equivalents. Representative examples of 5(47/)-oxazolones prepared via Michael additions are shown in Table 7.18 (Fig. 7.20). 

Selenol esters are useful as active acyl equivalents, particularly with regard to macrocycle formation. Equation (21) outlines the conversion of esters to selenol esters via aluminum reagent (48). ... 

A general synthesis of cyanoethyl ketones, in which a-amino-acids function as nucleophilic acyl equivalents, is outlined in Scheme 1... 

Similar methodology is used for the coupling of alkenyl halides and triflates with 1) alkenyl-, aryl-, or alkynylstannanes, 2) alkenylzinc species, or 3) arylboron species. This methodology is applied in the synthesis of cephalosporin derivatives (eq 7), and can be used for the introduction of acyl and vinylogous acyl equivalents (eq 8). 

Homoenolate equivalents (d reagents) are less conunon than nucleophilic acyl equivalents. The uses of 1-trimethylsilylallyl alcohols as effective homoenolate equivalents have been discussed. y-Alkylation of the trimethylsilyloxy-stabil-ized allylic anion formed by Brook rearrangement leads to the (isolable) tri-methylsilylenol ether (18) (Scheme 16). y-Alkylation of the functionalized ally-loxy-carbanion (19) has been reported. ... 

Amidrazones are new precursors of a-lithioenamines, which act as nucleophilic acyl equivalents (Scheme 8). The method appears to be limited to precursors derived from acids bearing an a- methylene group. 

A third term which has become useful for the discussion of synthetic analysis and planning is synthon. This term refers to a structural unit which has the potential for some specific synthetic operation. Again using the example above, we are searching for a group which would serve as a nucleophilic acyl synthon that is, some structural entity that would correspond to the addition of a nucleophilic acyl equivalent to an electrophilic carbon-carbon double bond. 

Sulfur compounds have also proven useful as the basis for nucleophilic acyl equivalents. The first reagent to find general use was 1,3-dithiane, which on lithiation provides the nucleophilic acyl group masked as a thioketal. The lithium derivative is reactive toward alkyl halides and carbonyl compounds." Closely... 

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