The total syntheses of the necine base (±)-platynecine and of a new indolizidine were accomplished. Both syntheses feature a [2+2] cycloaddition of a common endocyclic enecarbamate with alkylketenes in a new and concise strategy for the construction of pyrrolizidine and indolizidine skeleta. These syntheses were carried out in four steps in good overall yields. © 1995.362951095112Michael, (1994) Nat. Prod. Rep., pp. 17-39. , Indolizidine alkaloids:Michael, (1994) Nat. Prod. Rep., pp. 639-657. , and references cited thereinRobins, (1994) Nat. Prod. Rep., pp. 613-619. , Pyrrolizidine alkaloids:, and references thereinOlden, Breton, Grzegorzewski, Yasuda, Gause, Oredipe, Newton, White, (1991) Pharmac. Ther., 50, p. 285Suffiness, Cordell, (1985) The Alkaloids: Chemistry and Pharmacology, 25. , A. Brossi, Acad. Press, New York, chapter 1Faria, Matos, Correia, (1993) Tetrahedron Lett., 34, pp. 27-30At first we considered an ionic-stepwise mechanism the major operating mechanism for the [2+2] cycloaddition reaction of endocyclic enecarbamates and enamides to ketenes.4 This assumption was based on the results obtained by Huisgen for the [2+2] cycloaddition of enamines to ketenes (Otto, P,Feiller, L. A.Huisgen, R. Angew. Chem., Int. Ed. Engl. 1968, 7, 737–738.). However, attempts to perform [2+2] cycloaddition of endocyclic enecarbamates to alkylketenes in more polar solvents (CH2Cl2, THF) led only to decomposition. The results obtained so far in our studies with endocyclic enecarbamates indicates these as activated double bonds, but still retaining some of the enamines characteristic reactivity. Similar observation was made previously by Lenz for enamides (Lenz, G. R. Synthesis 1978, 489–518.)Krow, (1993) Org. React., 43, pp. 251-798. , For a comprehensive review on the Baeyer-Villiger reaction see:, see alsoKrow, (1991) Comprehensive Organic Synthesis, 7, pp. 671-688. , S.V. Ley, Pergamon Press, Oxford, For examples of the influence of conformational factors on the regioselectivity of Baeyer-Villiger reactions in α-substituted cyclohexanones see:Chida, Tobe, Ogawa, (1994) Tetrahedron Lett., 39, pp. 7249-7252Beauhaire, Ducrot, Malosse, Rochat, Ndiege, Otieno, (1995) Tetrahedron Lett., 36, pp. 1043-1046Ohmiya, Kubo, Otomasu, Saito, Murakoshi, (1990) Heterocycles, 30, pp. 537-542. , For a recent synthesis of tashiromine see:Paulvannan, Stille, (1994) J. Org. Chem., 59, pp. 1613-1620All new compounds gave satisfactory spectroscopic data. Representative data for some new compounds: 9HCl salt, sublimes above 200 °C: 1H NMR (300 MHz,D2O) δ: 4.90 (m, 1H), 4.45 (t,1H), 3.70 (t, 1H), 3.45 (dd, 1H), 3.40-3.10 (m, 3H), 2.35 (dd, 1H), 2.30-2.00 (m, 2H), 1.90-1.60 (m, 3H)13 C NMR (75 MHz, D2O) δ: 178.8, 74.7, 59.5, 51.2, 46.7, 46.4, 27.2, 22.7, 11.0IR (KBr): 2467-2411, 1760 cm−1MS (m/z): 176 (M+), 166, 149, 129, 97, 82, 69 (100%)10, 1 H NMR (300 MHz, CDCl3) δ: 4.19 (s, 1H), 3.90-3.50 (m, 5H), 3.20-3.00 (m, 2H), 2.35 (m, 1H), 2.20-1.35 (m, 8H)13 C NMR (75 MHz, CDCl3) δ: 69.2, 65.7, 63.3, 54.1, 54.0, 41.9, 31.8, 24.8, 19.5IR (neat): 3329, 2937, 2794 cm−1MS (m/z) 171(M+), 170, 140, 127, 114, 96 (100%), 68, 41Viscontini, Buzek, �ber Pyrrolizidinchemie. 11. Mitteilung [1]. Totalsynthese von (�)-Platynecin (1972) Helvetica Chimica Acta, 55, pp. 670-674Rüeger, Benn, The Enantioselective Synthesis of (+)-Retronecine, (—)-Platynecine, and (+)-Croalbinecine and Its C-1 Epimer (1983) HETEROCYCLES, 20, pp. 1331-1334Niwa, Kuroda, Yamada, A convenient synthesis of (.+-.)-retronecine. (1983) Chemistry Letters, pp. 125-126