Stable chiral spirocyclic [5,5]-ammonium ylides using a metallo carbenoid approach Daniele Muroni, a Antonio Saba a, * and Nicola Culeddu b a Dipartimento di Chimica, Facolta ` di Scienze, Universita ` di Sassari, Via Vienna 2, 07100 Sassari, Italy b CNR Istituto di Chimica Biomolecolare Sez. di Sassari, Via La Crucca, Baldinca- Li Punti (Sassari) 07040, Italy Received 14 September 2005; revised 21 October 2005; accepted 10 November 2005 Available online 2 December 2005 Abstract—Enantiomerically pure spiro[5,5]-ammonium ylides were obtained by Rh(II)-catalyzed decomposition of a-diazo-b- carbonylesters. In the crude decomposition mixtures, variable quantities of enamino-a,b-keto esters were detected as secondary products. q 2005 Elsevier Ltd. All rights reserved. 1. Introduction Recently, we prepared enantiopure indolizidinone alkaloids 2 by a carbenoid/spiro[5,5]-ammonium ylide/Stevens [1,2]- shift with ring-expansion tandem sequence 1 (Scheme 1). In this contest, isolation and characterization of ylide intermediates 1 in a stable and enantiopure form, enabled us, by studying their reactivity, to probe unambiguously the complete reaction cascade pathway previously proposed for similar processes, including the stereochemical reaction course. 2 We proved high chirality transfer from the original proline template to a second temporary spirocyclic ammonium nitrogen through its stereoselective quaternari- zation. The second one permitted the preservation of the original stereocenter during its [1,2]-shift to the newly formed neighbour quaternary indolizidinone stereocenter, a synthetic application of the SRS (self-regeneration of stereocenters) Seebach principle. 3 In some related studies involving metallo carbenoid generation and Stevens [1,2]-rearrangement of similar non-isolable ylide intermediates, a high degree of chirality transfer from the ammonium N atom was previously observed. 2 This stereoselectivity is in line with the extensive investigations of Ollis that demonstrate a solvent caged radical pair Stevens [1,2]-rearrangement reaction mechanism 4 or with an alternative recently proposed ion pair mediated mechanism. 5 Concerning the Stevens [1,2]-rearrangement, notwithstand- ing the considerable examples performed in related studies, there are few synthetic applications. One limitation of this methodology is the need for activating groups on the migrating carbons, such as allyl, aryl, carbonyl 6 and silyl; 2c consequently, no examples of a primary carbon migration have been reported, except for a few cases. 7 In view of the mild conditions, the carbenoid route seems most suitable for intramolecular ylide trapping, and given the rarity of stable spirocyclic ammonium ylides obtained 0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2005.11.020 Tetrahedron 62 (2006) 1459–1466 N O R CO 2 Et N H R N CO 2 Et O [1,2] 2 1 R R=COOR' Scheme 1. Keywords: Diazocompounds; Catalytic decomposition; Cascade process; Rearrangement. * Corresponding author. Tel.: C39 079 229538; fax: C39 079 229559; e-mail: saba@uniss.it