Six- and ®ve-membered 3-alkoxy-2-lithiocycloalkenes: new stable non-anionic b-functionalised organolithium compounds Miguel Yus, p,² Diego J. Ramo Ân and Inmaculada Go Âmez Departamento de Quõ Âmica Orga Ânica, Facultad de Ciencias, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain Received 10 January 2002; accepted 19 April 2002 Abstract ÐNaphthalene-catalysed reductive lithiation of various functionalised chlorocycloalkenes 18 leads to the corresponding non- anionic b-alkoxyfunctionalised organolithium reagents 14. Their reaction with different electrophiles, such as water, aldehydes, ketones and imines, gave the expected products 19 and 24. The diastereoselection in the reaction with aldehydes can be modi®ed by the use of different additives. In the case of using 3-methoxy-2-chlorocyclopentene 18a) as starting material, and depending on reaction time, unexpected bicyclopentadiene derivatives 25 were isolated, together with the expected compounds 24. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Functionalised non-stabilised organolithium compounds are interesting intermediates for the construction of organic structures due to the fact that their reactions with electro- philes usually lead directly to polyfunctionalised molecules. Therefore, in the last years, a great effort has been done in the development of highly functionalised organolithium reagents. 1 Their stability depends strongly on three factors. a) The type of functionality, so for example, the stability of different organolithium compounds decreases when the functionality has a good leaving-group character, the organolithium intermediate with anionic functionalities being more stable than the corresponding ones having a neutral functionality compare structures 1 and 2). b) The relative position between the functional group and the carbon±lithium bond, so the proximity between both moieties n0, 1, 2, 3, ¼ in structures 1 and 2) makes easier the corresponding a, b, g, d elimination reaction to give the corresponding carbene, alkene or cycloalkanes, respec- tively. c) The hybridisation of the carbanionic atom: it is generally known that the stability of a carbanionic inter- mediate follows the series sp.sp 2 .sp 3 . Due to the former considerations, organolithium compounds possessing a non-anionic leaving group in b position to the anionic carbon atom remain notoriously elusive 2 due to their great tendency to undergo b-elimination reactions to yield alkenes. 3 In the literature, there are only a few examples of this kind of organolithium species which are relatively stable. In the case of an amino-leaving group, the structures are of type 3, 4 4, 5 5 5 and 6, 6 while in the case of alkoxy- leaving group there are more different types of structures, such as 7, 7 8, 8 9, 9 10, 10 11, 11 12 12 and 13. 13 In spite of these examples, the different factors that prevent the elimination reactions in the aforementioned remarkable stable inter- mediates have not been de®nitely established. Thus, for example, in the case of intermediates 3 and 5, the stability may arise from the presence of other chelating function- alities. In other cases, the cyclic nature of some of them, such as 4, 5, 7 ± 9, 11 and 13 may prevent the appropriate conformation for the b-elimination reaction. Finally, it must be pointed out that in other cases, the stability may be due to the hybridisation either of the anionic carbon atom see reagents 6, 8 ± 12), or of the carbon atom which bears the functionality see intermediate 13). With this in mind, and due to the lack in the preparation of most simple organolithium intermediates of type 14, we anticipated that the preparation of this kind of intermediates could be carried out using a chlorine±lithium exchange cata- lysed by an arene 14 and they could be useful in organic synth- esis. Furthermore, the presence of an extra allylic ether group introduces the opportunity of further reductive cleavage of this functionality to yield a new allyllithium intermediate. 15 2. Results and discussion The starting chlorinated cycloalkenes compounds 18 were Tetrahedron 58 2002) 5163±5172 Pergamon TETRAHEDRON 0040±4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S0040-402002)00454-4 Keywords: lithiation; elimination reactions; lithium and compounds; cycloalkenes. p Corresponding author. Tel.: 134-9-6590-3548; fax: 134-9-6590-3549; e-mail: yus@ua.es ² www.ua.es/dept.quimorg