A new approach to optically active N-methylamino acids is presented, relying on stereoselective reduction of N-methylisoxazolinium salts with a dioxyethyl side-chain. The diastereoselectivity of the reduction step is studied systematically, in comparison with that of respective isoxazolines. A two-step transformation of isoxazolinium salts - with NaBH3(OAc) and subsequent catalytic hydrogenation as well as a one-pot reduction by catalytic hydrogenation led to high (95:5 and 87:13) diastereomeric ratios of protected erythro-N-methylaminopentanetriols. The hydroxyethyl side-chain is elaborated by oxidation to afford the β -N-methylamino acid 37, exemplifying the potential of this strategy.

Dedicated to Professor Alessandro Dondoni on the occasion of his 75 th birthday2-Isoxazolines represent a well known class of heterocycles, readily accessible in particular by 1,3-dipolar cycloaddition of nitrile oxides to alkenes. 2-Isoxazolines are easily transformed into 2-isoxazolinium salts by N-methylation, and further into 3-isoxazolines by deprotonation. In contrast to the parent system, less is known concerning the chemistry of the derived classes, and potential applications in synthesis. -2-Isoxazolinium salts, due to their iminium part, are prone to the attack of nucleophiles, and examples for this, addition of hydride (reduction) and C-nucleophiles like methylmagnesium bromide, cyanide, methane nitronate, and malonate are given. With these adducts, syntheses of β -and α-amino acids with OH-containing side chains have been effected. The cyanide products also are useful as precursors of branched, unsymmetrical 1,2-diamino polyols. -On the other hand, 3-isoxazolines due to their oxy-enamine part, represent species with nucleophilic sites and therefore react with electrophilic reagents. Examples given are [3+2] cycloadditions with nitrile oxides, [2+2] cycloadditions with dimethyl acetylenedicarboxylate, and [2+1] cycloaddition in the form of epoxidation which, however, led to a dihydro-1,3-oxazine nitrone by initial attack at the nitrogen atom, in an unprecedented oxidation/N-dealkylation/rearrangement sequence.

C13H24BF4NO3, orthorhombic, C222i (no. 20), a = 9.320(2) Ä, b = 10.6761(8) A, c = 33.786(2) Ä, V= 3361.6 Ä 3 , Z= 8, Rgi(F) = 0.064, wR^F 2 ) = 0.185,7= 293 K. Source of materialThe title compound has been obtained by 1, DiscussionThe isoxazolidine ring system shows an envelope conformation (figure, top). Three atoms (C10,CI 1,C12) of thecyclohexylidene moiety exhibit very large displacement parameters. A resolution of eventual disorder via split positions was not performed, the distances of the C-C bonds deviate less than 5 % from normal values. The fluorine atoms of the tetrafluoroborate anion have also large displacement parameters, which indicates considerable rotation of the anion. The packing diagram reveals a bilayer-type stacking of the molecules with alternating polarity along the c axis ( figure, bottom). The non-polar bilayer is built up by a face-to-face orientation of the cyclohexylidene moieties; the polar bilayer is built by the hetero atoms of the isoxazolidines and the tetrafluoroborate anions.

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