Methyl-2- trimethylsilyloxy -3-pentanone

The heteroaromatic strategy is extended to the synthesis of fu-rans when acylium ions are employed as the heteroallenophile (eq 16). Acylium ions are generated in situ via the reaction of acyl chlorides and aluminum chloride. Typically, the allenylsilane is added to a solution of 1.0 equiv each of AICI3 and the acyl chloride in methylene chloride at — 20 °C. The reaction is complete in 1 hat-20°C. 

Synthesis of Azulenes. Reaction of tropylium cations with allenylsilanes produces substituted azulenes. Typically, commercially available tropylium tetrafluoroborate (2 equiv) is ert5)loyed. The second equivalent dehydrogenates the dihydroazu-lene intermediate to produce the aromatic product. Poly(4-vinyl-P3Tidine) (poly (4-VP)) or methyltrimethoxysilane is used to scavenge the HBF4 produced in the reaction. 

For reviews on allylsilanes and related systems, see Ref. 1 and Fleming, I. Dunogues, J. Smithers, R., Org. React. 1989, 37, 57 Heming, I., Comprehensive Organic Synthesis 1991,2,563. 

For reviews of the chemistry of propargylic anion equivalents, see (a) Yamamoto, H., Comprehensive Organic Synthesis 1991, 2, 81. (b) Epsztein, R. In Comprehensive Carbanion Chemistry Buncel, E. Durst, T, Eds. Elsevier Amsterdam, 1984 Part B, p 107. (c) Moreau, J.-L. In The Chemistry of Ketenes, Allenes, and Related Compounds Patai, S., Ed. Wiley New York, 1978, p 343. 

Katherine L. Lee Rick L. Danheiser Massachusetts Institute of Technology, Cambridge, MA, USA 

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Still s synthesis of monensin (1) is based on the assembly and union of three advanced, optically active intermediates 2, 7, and 8. It was anticipated that substrate-stereocontrolled processes could secure vicinal stereochemical relationships and that the coupling of the above intermediates would establish remote stereorelationships. Scheme 3 describes Still s synthesis of the left wing of monensin, intermediate 2. This construction commences with an aldol reaction between the (Z) magnesium bromide enolate derived from 2-methyl-2-trimethylsilyloxy-3-pentanone (21) and benzyloxymethyl-protected (/ )-/ -hydroxyisobutyraldehyde (10).2° The use of intermediate 21 in aldol reactions was first reported by Heathcock21 and, in this particular application, a 5 1 mixture of syn aldol diastereoisomers is formed in favor of the desired aldol adduct 22 (85% yield). The action of lithium diisopropylamide (LDA) and magnesium(n) bromide on 21 affords a (Z) magnesium enolate that... 

Overall 1,4-diastcreoselection can be achieved by a sequential aldol addition-Claisen rearrangement. For example, alcohol 11 obtained in >50 1 diastereoselectivity from the reaction of crotonaldehyde and 2-methyl-2-trimethylsilyloxy-3-pentanone (9) via 10 was subjected to the Meerwein-Eschenmoser variant to give a 9 1 mixture of amides 12 and 13384. 

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