A-isocyanocarboxylates

The gold(I) complex of a chiral ferrocenylphosphine complex promotes asymmetric aldol reactions of a-isocyanocarboxylates to form chiral oxazolines in high diastereo- and enantio-selectivities (Scheme 52).225,226 In these reactions, the analogous silver(I) ferrocenylphosphine complex also works well. 

As shown in Scheme 3-30, a,/ -unsaturated aldehyde is treated with a-isocyanocarboxylate in the presence of gold complex (S)-(R)-82 to afford, after several steps, D-eryt/zro-sphingosine derivative.53b... 

B1.2. GOLD-CATALYZED ASYMMETRIC ALDOL REACTION OF a ISOCYANOCARBOXYLATES 495... 

The catalytic asymmetric aldol reaction is tolerant of a-isocyanocarboxylates bearing an a-alkyl substituent (9a-d) as shown in Scheme 4 and Table 8B1.4 [17,18]. The enantioselectivities in the reaction with paraformaldehyde are generally moderate, with the exception of the... 

TABLE 8B1.4. Gold-Catalyzed Asymmetric Aldol Reaction of a-Isocyanocarboxylates 9 with Aldehydes (Scheme 8B1.4)... 

Gold and Silver-Catalyzed Asymmetric Aldol Reactions of a-Isocyanocarboxylates... 

The required a-isocyano derivatives (85) of a-amino acids are prepared from the corresponding a-formylamino acids. If (85) is a peptide derivative, no racemization is observed at the a-isocyanoacyl unit. - The preparation of chiral a-isocyanocarboxylate esters, however, is possible with phosgene or... 

The same issue is addressed in Table 8, in which reactions of nonracemic substrates are described. Regioselectivity is under steric control. All four acetates 49-52 provide mainly the ( -enantiomer82-83, probably through a common syn,syrt-n-a] yl complex. In most cases, almost 100% chirality transfer is observed. The a-isocyanocarboxylates (Table 8, entries 9, 10)84 are interesting nucleophiles for the preparation of amino acids easily obtained from the alkylated products after acidic hydrolysis. This sequence constitutes an alternative to the Schiff bases of glycine already discussed (see Table 4). Moreover, radical reduction of the isocyano group with tributylstannane yields the a-unsubstituted carboxylic esters. 

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