No abstract

791Among the reaction sequences for the synthesis of terpene lactones and derivatives the functionalization of an existing ring system with appropriate functional groups is less explored [1 -7]. Whereas syntheses based on intramolecular Diels-Alder reactions [1] or cascade radical processes [2] starting from a functionalized acyclic molecular ensemble bearing an ester functional group or a butenolide are well established, minor attention has been turned to annelative ring expansion strategies. However, starting from cyclic ketones syntheses of annulated butenolides have been accomplished [3], e.g. by Pummerer reaction of γ-(phenylsulfinyl)-α,β-unsaturated aldehydes [4] or group transfer cyclization reactions [5]. In addition lithiation-carboxylation strategies [6] and palladium-catalyzed, carbonylative intramolecular coupling reactions [7] with hydroxy vinylbromides or hydroxy trifluoromethanesulfonates have been applied.Our approach is based on the application of cyclic dicarbonyl(cyclopentadienyl)iron-substituted enals 5 synthesized from an existing ring system with a ketone functionality, for the construction of either lactones or lactams by one-pot procedures [8 -12]. Recently, we reported on the synthesis of α,β-butenolides 6 from β-[η 5 -C 5 H 5 (CO) 2 Fe]-substituted enals 5 and organolithium or Grignard reagents [11,12].We herein wish to report full details on observations with organolithium compounds. Moreover, the experimental procedures for the synthesis of cyclic β-[η 5 -C 5 H 5 (CO) 2 Fe]-substituted enals 5 from β-keto esters 1 are described [12]. Abstract.A reaction sequence furnishing cyclic β-[η 5 -C 5 H 5 (CO) 2 Fe]-substituted enals 5 starting from β-keto esters 1 is described. Organolithiums were found to react smoothly with the iron-substituted enals yielding α,β-butenolides 6 by cyclic β-[η 5 -C 5 H 5 (CO) 2 Fe]-substituted enals 1 with aliphatic substitution pattern (Scheme 1). The cyclohexene carbaldehyde complexes 5a and 5b and the 5α-cholestan-3-one-derived compound 5c were synthesized by this route. Major limitations of the halo Vilsmeier reaction of ketones, a method conveniently employed in the synthesis of iron-substituted enals when starting from β-halovinyl aldehydes, became obvious when applied to cyclic annelated ketones with aliphatic substitution pattern, especially steroidal ketones [13]. For example, starting from 5α-cholestan-3-one by dropwise addition of the Vilsmeier-Haack reagent prepared from POCl 3 and DMF (molar ratio 7:8) at 0 °C and subsequent heating to reflux for 3 -4 hours a separable 2:1 mixture of the regioisomeric β-chloro enals described in the experimental section was only obtained in 54% overall yield [14].an intramolecular cyclocarbonylation of the lithiumalkoxide initially formed. The influence of e.g. the reaction temperature and the solvent on the reaction cascade is discussed. A reaction mechanism is proposed.

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