Addition of ally1 alcohol to 3,4-dihydro-2H-pyran 2 in the presence of traces of hydrochloric acidr5] affords the acetal 3 (yield: 85%). Dibromocarbene, generated by phase-transfer catalysis, adds to the double bond of 316] with formation of two diastereomers 4a (ratio ca. 1 : 1, yield: 64%). Reaction of 4a with methyllithium in ether at -75°C leads to formation of the tricyclic spiroacetal 6[61 as main product (20%, not yet optimized), along with the bromocyclopropane compounds 4c (ca. 1 5%)I6.'l and the methylated diastereomers 4d (ca. 5%).The tricycle 6 is the product of an intramolecular insertion of the cyclopropylidene(oid) into the tertiary C-H bond of 5 . Dreiding models show that this ''activated"'81 C-H bond, flanked by two oxygen atoms, is located in a favorable position for the insertion. Insertions into C-H bonds adjacent to a heteroatom (OI9], N, S) with formation of a five-membered ring are well-known[*]. Compounds resulting from insertion into secondary C-H bonds in the aposition to the oxygen atoms are, according to the mass spectrum of the crude mixture (coupled GC-MS), not formed. Accordingly, 6 is the only product formed on reaction of the carbene(oid) 5['01. The exclusive formation of 6, in which the cyclopropane ring and the oxygen atom of the six-membered ring are located on the same side of the molecule['01, could be due to the fact that no steric interaction of the geminal three-membered ring protons on C-4 with the protons on C-11 takes place in the transition state of the insertion reaction. This stereoselectivity could also be favored by an intramolecular complexation in 4b. After coordination of the lithium by the oxygen atoms in 4b and subsequent LiBr cleavage, the resulting carbene(oid) 5 can insert into the tertiary C-H bond.No allene could be detected by IR and 'H-NMR spectroscopic examination of the crude mixture formed in the reaction of 4a. The two oxygen atoms in 5 would therefore also appear to prevent a competing cyclopropylidene-allene rearrangement at -75"C19b."1. On the other hand, reaction of 4a at room temperature leads to formation of a n allene as well as 6. 4c and 4d could be formed by protonation and methylation of 4b, respectively"'. The hydrogenation"'I of 6 (10% Pd on activated charcoal, CH30H, 5 bar, 25 "C) affords 3-and 4-methyl-1,6-dioxaspiro[4.5]decane 7 and 8, respectively (1.5 : 1) and the natural product 1[5.'31 in an overall yield of 80% (7 + 8 : 1 = 1.75 : 1). Registry numbers:1, 95248-66-1; R-( +)-Et,P(S)NCH(Me)Ph-Li', 95217-04-2; NiCI,, 7718-54-9; R-( +)-P,P-diethylthiophosphinic acid-N-a-metbylbenzylamine, 952 17-05-3; diethylthiophosphoryl bromide, 398 1-46-2; R-( +)-a-methylbenzylamine, 3886-69-9; n-butyllithium, 109-72-8.[ I ] H. resonances of one CH2 group can be exchanged, as also A, J, H and B, I, G. 232 [3] R. Benn, H . Giinther, Angew. Chem. 95 (1983) 381; Angew. Chem. I n l . Ed. Engl. 22 (1983) 390. 0 VCH Verlagsgesellrchaft mbH, 0-6940 Weinheim 1985 0570-0833/85/0303-0232 S 02.50/0 Angew. Chem. Int. Ed. Engl. 24 (1985) No. 3
