NMR and X-ray crystallographic studies of axial and equatorial 2-ethoxy-2-oxo-1,4,2-oxazaphosphinane Irma Linzaga, a,b Jaime Escalante, b Miguel Mun ˜oz b and Eusebio Juaristi a, * a Departamento de Quı ´mica, Centro de Investigacio ´n y de Estudios Avanzados del Instituto Polite ´cnico Nacional, Apartado Postal 14-740, 07000 Me ´xico, DF, Mexico b Centro de Investigaciones Quı ´micas, Universidad Auto ´noma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62210 Cuernavaca, Mor., Mexico Received 10 July 2002; revised 9 September 2002; accepted 12 September 2002 Abstract—Condensation of (S)-(N-benzyl)-phenylglycinol with formaldehyde in the presence of diethylphosphite afforded hydroxy- aminophosphonate (S)-3, which upon treatment with KH yielded a 97:3 mixture of the interesting, phosphorus-containing 2-ethoxy-2-oxo- 1,4,2-oxazaphosphinanes (2S,5S)-1 and (2R,5S)-1. Suitable crystals of both diastereomers were used in X-ray crystallographic studies that permitted unequivocal configurational assignment, as well as examination of the consequences of n O !s P–O p stereoelectronic interactions on structural properties. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Some years ago, Dellaria, Williams, and co-workers 1 developed chiral glycine derivatives A and B as convenient precursors in the enantioselective synthesis of a-substituted a-amino acids. Inspired by this work, and motivated by the large range of biological activities presented by the analogous a-phosphonic acids, 2,3 Royer et al. recently reported the preparation of oxazaphosphinane derivative C. 4 Nevertheless, compounds C were obtained as a 4:1 diastereomeric mixture of epimers at phosphorus, that was not separated into the pure epimers. In this paper, we describe a highly diastereoselective synthetic procedure for the preparation of enantiopure (2S,5S)-4-benzyl-2-ethoxy-2-oxo-5-phenyl-1,4,2-oxaza- phosphinane [(2S,5S)-1] from (S)-phenylglycinol (Scheme 1). 0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(02)01152-3 Tetrahedron 58 (2002) 8973–8978 Scheme 1. * Corresponding author. Tel.: þ52-55-5747-3722; fax: þ52-55-5747-7113; e-mail: juaristi@relaq.mx Keywords: phosphorus heterocycles; diastereoselection; NMR; stereoelectronic effects.