TETRAHEDRON LETTERS Tetrahedron Letters 42 (2001) 7809–7813 Pergamon New methodology for the [4+3] cycloaddition reactions: generation of oxyallyl cations from ,-diiodoketones under sonochemical or thermal conditions Angel M. Montan ˜a* and Pedro M. Grima Departamento de Quı ´mica Orga ´nica, Universidad de Barcelona, Martı ´ i Franque `s 1 -1, 08028 Barcelona, Spain Received 31 August 2001; revised 5 September 2001; accepted 6 September 2001 Abstract—A new methodology to perform [4+3] cycloaddition reactions of suitable dienes and 1,3-dimethyl-2-oxyallyl cations is presented. The reaction is carried out starting from commercially available dienes and easy-handling ,-diiodoketones, which are reduced by the Zn/Cu couple to generate the oxyallyl cation as intermediate. The reaction is carried out under mild thermal or sonochemical conditions at low temperatures (from 0 to -44°C) and for short reaction times (<15 min). This methodology represents a good alternative respect to the actual procedures based on other reducing agents or starting from more elaborate precursors of the oxyallyl cations. © 2001 Elsevier Science Ltd. All rights reserved. The work presented deals with the development of a new methodology for the [4C(4)+3C(2)] cycloaddi- tion reaction 1 of dienes with oxyallyl cations, generated in situ from dihaloketones in the presence of reducing metals (Scheme 1). This work is a part of a wider study that we are carrying out in our laboratory on the [4+3] cycloaddition reaction of heterocyclic dienes 2–4 and 1,3- dimethyl-2-oxy-allyl cation, in order to synthesize poly- functionalized cycloheptanes (Scheme 2). These molecules are important precursors of both versatile linear and cyclic synthons 5 for the preparation of bio- logically active natural products. 6,7 Scheme 1. Diastereomeric products resulting from the [4+3] cycloaddition reaction of dienes and 1,3-dimethyl-2-oxy-allyl cation. Scheme 2. Synthetic applications of the [4C(4)+3C(2)] cycloadducts. Keywords : [4+3] cycloaddition; oxyallyl cation; diiodoketone; Zn–Cu couple; sonochemistry. * Corresponding author. Fax: +34-93-3397878; e-mail: ammontana@qo.ub.es 0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved. PII:S0040-4039(01)01671-9