threo‐3‐Alkyl‐ and ‐Arylglutamic Acid Derivatives by Michael Additions of Boc‐BMI Li‐Enolates to 2,6‐Di‐t‐butyl‐4‐methoxyphenyl Alkenoates on the Diastereoselectivity of the Coupling of Trigonal Centers Involving Heterocyclic Li‐Enolates

The conjugated esters 5 , 6 , 8 , 9 mentioned in the title are prepared from the corresponding aryl acetate 4 and aldehydes by aldol condensation (direct enoylation of the hindered phenol with the corresponding unsaturated acid chlorides gave only poor yields). To avoid double‐bond shifts, the 4‐phenyl 2‐butenoate 7a was prepared from the saturated one by selenation/elimination (see 7b , c in Scheme 1). In contrast to methyl and ethyl crotonates, reacting in poor yields and with low selectivities (→ 11 , 12 ), the hindered aryl enoates combine with the Li‐enolate of 1 [( S )‐Boc‐BMI, a chiral glycine derivative] to give single products 13a – 17a (⩾95% ds) in high yields (78–96% of purified samples, Scheme 2). The configuration of the two newly formed stereogenic centers and thus the mode of coupling of the trigonal centers in the Michael addition ( D and E in Scheme 4) is derived for the methyl ( 13a ) and benzyl substituted ( 15a ) derivatives by multiple chemical correlation (see 19 – 24 in Scheme 3). It is shown that pure (2 S ,3 R )‐glutamic acids 22 , 23 can be isolated by hydrolysis of the adducts 13a, 15a of conjugate addition. — The results of the aldol, Michael and nitro olefin additions of heterocyclic Li‐enolates are collected and compared (Scheme 5, Table 1). Possible reasons for the high diastereoselectivity and a typical coupling mode of most reactions involving N ‐acylimidazolidinone and N ‐acyloxazolidinone Li‐enolates M , O are discussed (see W in Scheme 6).