N-allyl aldimines

Fig. 6.10 Corey s selectivity model for the Strecker reactions of N-allyl aldimines.

Asymmetric synthesis of fi-amino acid esters. The chiral diazaborolidinc 1 (16,155) also effects diastereo- and cnantioselective reactions of (S)-/-butyl thiopro-ponoatc (2) with N-bcnzyl or N-allyl aldimines 3 to form /8-amino acid esters 4, precursors to chiral tnw.r-/3-lactams (5) in 90-99% cc. 

The synthesis of vinylaziridines through reactions between allylic carbenoid reagents and imines (i.e., Darzen-type reactions) was first reported by Mauze in 1980 [13]. Treatment of aldimines or ketimines 16 with gem-chloro(methyl)allyllithium (17) afforded N-substituted vinylaziridines 18 (Scheme 2.6). Similarly, 2,3-trans-N-diphenylphosphinyl-2-vinylaziridines 21 were prepared with good stereoselectivities (trans cis= 10 1 Scheme 2.7) by treatment of a-bromoallyllithium (20) with N-diphenylphosphinyl aldimines 19 in the presence of zinc chloride [14]. 

In previous work, Corey used the free base form of 34 as an effective chiral ligand in the Os04-promoted dihydroxylation of olefins [90]. He later found that ammonium salt 34 catalyzed the addition of HCN to aromatic N-allyl imines (Scheme 5.50) [91]. The U-shaped pocket of the catalyst is essential in fixing the orientation of the hydrogen-bonded activated aldimine via n-n interactions. 

Scheme 6.40 Product range of the 11-catalyzed asymmetric Strecker reaction of aromatic and aliphatic N-allyl-protected aldimines.

In the presence of 42 (2mol% loading), aliphahc and aromafic N-allyl as well as N-benzyl aldimines were efficiently converted after 20 h at -70 °C in toluene to the respective Strecker adducts and subsequently trifluoroacetylated to obtain the products 1-10 in good to excellent yields (65-99%) and ee values J7-97%) (Scheme 6.41). It turned out that N-benzyl imines could be used as substrates without significant difference in comparison to analogous N-allyl imines (e.g., N-benzyl adduct 8 85% yield, 87% ee N-allyl adduct 9 88% yield, 86% ee Scheme 6.41). 

The allylation of hydrazones derived from arylalkyl ketones [42] and of N-aryl aldimines in DMF at 0 °C [46] have also been reported. 

Some examples of combined syn-anti and diastereofacial selectivity employing /V-n-propyl- and N-isopropyl-aldimines, derived from a-phenylpropionaldehyde, and crotyl-9-BBN, -magnesium and -zirconium reagents have been reported by Yamamoto et al. Cram selectivity, which is observed in the analogous reactions of these chiral imines with allyl organometallics (see Section 4.3.2.1.2i), is preserved as ratios of Cram anti-Cram products are consistently about 8 1. Anti selectivity is also observed but the ratios do not exceed 7 3. The weak anti selectivity parallels that observed in reactions of crotyl-9-BBN with branched a-alkylaldimines. Since syn-anti selectivity is influenced more by the a-substituent than by the A-substituent of the aldimine, more synthetically useful levels of combined syn-anti and diastereofacial selectivity might be expected in other series of a-substituted aldimines. 

In the presence of a Lewis acid or fluoride ion, imines react with allylsilane to yield the homoallylic amines with high stereoselectivity119,120. Thus, treatment of N-galactosylaldimine 81 with allylsilane in the presence of excess of SnCU yields the corresponding allylated product 82 (equation 54). It is noted that aliphatic aldimines do not react under these conditions. Fluoride ion promoted crotylation of aldimines proceeds in a regiospecific and diasteroselective manner121. A pentacoordinate silicate moiety is involved in this reaction. 

Amino acid synthesis (8, 389). Alkylation of the aldimine (1) from glycine ethyl ester and /j-chlorobenzaldehyde under phase-transfer conditions offers a general route to amino acids. Either liquid-liquid phase-transfer or solid-liquid phase-transfer catalytic conditions are satisfactory with active halides, but alkylation with allylic halides and less active alkyl halides is best effected under ion-pair extraction conditions (6,41), with 1 equiv. of tetra-n-butylammonium hydrogen sulfate (76-95% yields).1... 

In a typical example for aziridine synthesis,, y-allyl tetrahydrothiophenium bromide 618 was smoothly deproto-nated with strong base to provide an ylide which adds to a variety of N-protected imines. For the iV-tosyl aldimine 619 derived from isovaleraldehyde, the corresponding vinyl aziridine 620 is formed in fair yield as a mixture of stereoisomers (Scheme 151) <2004TL1589>. 

Following their earlier findings on the enantioselective allylzincation of cy-clopropenone acetals [43], Nakamura and co-workers investigated the same reaction with imines of aromatic aldehydes [44]. In an initial experiment, ( )-ben-zaldehyde N-phenyl imine (59) was treated with preformed chiral allylic zinc reagent 57a (R=z-Pr) obtained by mixing a lithiated bis(oxazoline) (55, R=z-Pr) with an allylic zinc bromide (56a) (Scheme 22). The allylation product was obtained in 95% yield, but the enantioselectivity was only 6% ee. Fortunately, the enantioselective allylzincation of Z- (i.e., cyclic) aldimines was more successful. 

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