In the past few decades natural and non-natural prolines-and among the latter, a-quaternary derivatives are of great interest-have found wide application in the design of new organocatalysts [1] and chiral auxiliaries [2] for asymmetric synthesis. Furthermore, non-natural proline units have been incorporated in new peptide chains, affording peptidomimetics with different conformational flexibility and correlated drastic changes of their secondary structures. [3] In the best cases, these alterations enhance the resistance of the modified proteins to metabolic and chemical degradation and hence their overall biological activity.A number of strategies [4] -and, among them, those that adopt the well-known Kawabatas principles of "memory of chirality" (MOC) [5] and of "chiral non-racemic enolate" [6] emerge as the most appealing-have been proposed for the stereoselective synthesis of quaternary a-amino acids. In particular, the synthetic methods for quaternary prolines [7] involve the enantioselective functionalization of l-proline derivatives, for example, through self-reproduction of chirality, diastereoselective alkylation, or transfer of stereochemical information via cyclic ammonium ylides.Recently, we reported a straightforward protocol for the enantioselective synthesis of quaternary N-alkyl-a-4-nitrophenyl-a-amino tert-butyl esters, through N-alkylation of the corresponding a-N-(4-nitrophenyl)sulfonamido esters, followed by degradative rearrangement, with loss of SO 2 .[8]The one-pot overall process is highly stereoselective, ees up to 98 %; in contrast, the reaction conducted with the preformed N-alkyl-a-N-(4-nitrophenyl)sulfonamido ester gave the corresponding quaternary amino ester with very low ees, for example, 28 % ee in the case of N-allyl phenylalanine derivative. In the present paper we describe the rearrangement under basic conditions of N-(arylsulfonyl)proline tert-butyl esters 1, which showed a different behavior of that found for N-alkylated open-chain sulfonamido esters. Preliminary runs (Table 1), conducted on N-(4-nitrobenzenesul-[a] F.

The 'one-pot' stereoselective conversion of N-(4-nitrobenzene)sulfonyl-alpha-amino acid tert-butyl esters into the corresponding N-alkyl-alpha-(4-nitrophenyl)-alpha-amino esters has been realized through N-alkylation of the starting amido esters, followed by N-C(alpha) migration of the p-nitrophenyl group and the loss of sulfur dioxide; the asymmetric induction is determined by an intermediate non-racemic enolate, without the need of an external source of chirality.

Polyfunctionalized benzo[d]sultams 7 and 8, which contain an alpha-amino acid unit, have been synthesized from the corresponding open chain (pentafluorobenzene)sulfonamides 4 by complementary solid-liquid phase transfer catalysis (SL-PTC) and homogeneous protocols. The cyclization step proceeds through the intramolecular nucleophilic displacement of an aromatic fluorine atom.

A straightforward and efficient synthesis of 2-substituted 3,4-dihydro-2H-1,4-benzoxazines 6−9 has been carried out in excellent overall yields through the ring opening of epoxides 1 with arylsulfonamides 2 and 3, followed by cyclization of the hydroysulfonamides 4 and 5 thus obtained. Both steps are carried out under solid−liquid phase transfer catalysis (SL-PTC) conditions using potassium carbonate and sodium hydroxide as bases. The procedure is feasible for the synthesis of non-racemic 2-substituted benzoxazines.