Regiospecific bromination and debromination reactions have been used to provide a synthetic route from camphor to optically active 8,10- and 9,10-disubstituted camphor derivatives.
Certain lignans can be prepared using, as the key step, a thermal reaction between 1 -(aryl)-l,3-dihydro-5,6dialkoxy-benzo[c]thiophen 2,2-dioxides and dienophiles.
Novel routes to 1 -aryl-5,6-dialkoxy-l,3-dihydrobenzo[cJthiophene 2'2-dioxides (1 3) and to the corresponding benzo[c]furan (8) have been developed; these species yield quinodimethanes (1 2) via thermal extrusion of SO, and isobenzofurans (5c) in an acid-catalysed process, respectively, which then react with various dienophiles to provide structural analogues of the aryltetralin and arylnaphthalene lignans.
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Stereoselective allcylation of a bicyclic lactone (9) derived from (+)-camphor (5) is the basis of a synthetic route to bicyclic enone (3) which could be used as a key intermediate in helenanolide synthesis.I JOHN H. HUTCHINSON, THOMAS MONEY et SUSAN E. PIPER. Can. J. Chem. 64, 1404Chem. 64, (1986.On utilise l'alcoylation stCrCodlective de la lactone bicyclique 9, provenant du (+)-camphe (5), pour prCparer 1'Cnone bicyclique 3 qui peut &tre utilisCe c o m e intermaiaire clC dans la synthtse de l'hCl&nanolide.[Traduit par la revue]Pseudoguaianolides are sesquiterpenoid lactones which are classified as helenanolides (cf. helenalin (1)) or ambrosanolides (cf. ambrosin (2)) on the basis of their configuration at the C(10) position. The cytotoxic activity (1) and structural complexity of the helenanolides and ambrosanolides has stimulated considerable interest in their total synthesis and several imaginative routes to specific sesquiterpenoids of each class have been reported (2-13). A recent report (13) by Lansbury and co-workers describing the synthesis of a pseudoguaianolide intermediate by ring cleavage of 9-iodocamphor oxime prompts us to record our synthetic studies in this area.As part of our general investigations on the use of camphor as a chiral starting material in natural product synthesis (9) (14, 15) we have recently been involved in the development of simple synthetic routes from (+)-camphor (5) to epimeric dienones, 3 and 4, which have considerable potential as intermediates in the synthesis of ent-helananolides and ent-ambrosanolides, respectively.? The synthetic route to dienone (3) involves an intermediate bicyclic lactone (14, 15) which is readily synthesized from 9,lO-dibromocamphor (7) (14a)~ by the previously reported procedure (15) (cf. Scheme 1). Subsequent alkylation of 9 with methyl iodide provided the expected 4 @-methyl derivative ' TO whom all correspondence should be addressed. '~ienones (3) and (4) are synthons for the ent-helenanolides and ent-ambrosanolides, respectively. To synthesize enantiomers of (3) and (4) would involve C(3)-bromination (Br2/HOAc) of commercially availible (-)-camphor as the first step in the sequence. (-)-Camphor is relatively expensive and although this extra expense can be avoided to some extent by oxidizing commercially available (-)-borne01 with hypochlorite (16-18), the convenience of starting with (+)-3-bromocamphor (6) prompted us to conduct our initial investigations in the ent-series.3 0~r published (14a) yield (-60%) for the conversion of 3,9,10-tribromocamphor to 9,lO-dibromocamphor (7) has been increased to -90% by conducting the selective debromination (Zn/HOAc/ether) in an ice-bath for 30 min.10 as the only isolable product in -55% yield. The stereochemistry of the methyl group in this compound was supported by nmr evidence, i.e. the 4a-hydrogen signal, on irradiation with the C(4)-methyl resonance frequency, collapsed to a doublet with a coupling constant (J = 12 Hz) consistent with its anti relationship with the 5 @-hydrogen atom.4 Methanolys...
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