Table 3. C')cloaddirion of allylsilane 2b and methyl propynoate (13) Reaction conditions 14. Yield [%I 15, Yield [%I 3 equiv 2b. -7X C --20 C. 19 h 4equi\2h. -78 t -2 5 C . 5 d 45 46 [a] 98 -3 e q u i v Z h . X C -4 0 C . 1 9 h 34 64 [a] [a] ( i i i r r . \ i +IS: \ w , . \ j ~i i -l S = 3: 1. Traces of the Sakurai product and a cyclopentane dcriwtne were i i l w dcrected. tion at room temperature for fivedays. 14 and the bicyclo[2.2.0]hexane 15. the product of the domino reaction, were formed i n approximately equal amounts. Cycloaddition of 2 b with 13 in dichloromethane at reflux for 19 h provided 15 and 14 in yields of 64% and 34%, respectively. Bicyclo[2.2.0]hexane 15 was obtained as a mixture of the anti,sw and the s.vn,syn diastereomers in a 3:l ratio. The two diastereomers are easily distinguished by their I3C N M R spectra. The major isomer rmti.sjn-15 shows two signals for the aSi-CH2 groups, one anti to the methoxycarbonyl group at 6 =10.2 and one syn at 6 = 13.3. The minor isomer syrz.syn-15 is symmetrical and therefore exhibits only one signal for the aSi-CH, group in the region typical for the syn diastereoisomer (6 = 13.6). Thus, this product was assigned the syr,sjn configuration with both silylmethyl groups on the same side as the methoxycarbonyl group. Expcvinic~nral Procedure 15: A b o l u t i o n of methyl propynoate (13) (137 pL. 339 ing. 4.03 mmol) in dry dichloromethane ( 5 mL) was added to a stirred solution of titanium tetrachloride (490 pL. 840 mg. 4.43 inmol) in dry dichloromethane ( 5 mL) at room temperature. To this ini\ture was added a solution ofallyltriisopropylsilane (2b) (2.92 mL. 2.40 g.
13.08 miiio) i n dry dichloromethane ( 5 mL). The reaction mixture was refluxed for 19 h and tlieii quenched by addition of an aqueous solution ofamrnonium chloride.
The organic layer was separated, the aqueous layer was extracted three times with dichlorometh;rne. and the combined organic layers were dried over magnesium sulfatc. E;iporatioii of the solvent m d flash chromatography (hexane:ether 1 5 : l ) of the rcsidue o n silica gel provided the bicyclo[2.2.0]hexane 15 (1.24 g. 64"/0) and the cyclobutene 14 (387 mg, 34%) both as colorless oils.