mechanisms: (a) intermediate 3 or (b) another intermediate formed from 3 by loss of the leaving group is trapped by reaction with sulfite ion or thiamin. Consequently, simpler mechanism a is given in Scheme I. Incorporation of an additional step as required by pathway b does not change the kinetic form, only the collection of rate cons tan ts .The derivation pertains to experimental conditions3 where sulfite ion S is the limiting reagent under initial rate conditions and 1 is present in excess. The pH dependence is not expressed but it is to be understood that the conjugate acid of thiamin reacts with free sulfite ion while thiamin base is a nucleophile.Step k3 is not included in the rate expression because one S is consumed and another is liberated. Note that S is liberated in step k4 in which thiamin traps intermediate; the formation of oligomer is catalyzed by S, and so step k4 must be included in the rate expression. Applying a steady-state assumption for 3 gives eq 2. This has the form of eq 1 in ref 3, kz + k 4 [ l ] being an apparent constant.
The k2/k3 ratio in ref 3 now becomes (k, + k 4 [ l ] ) / k 3 and reduces to k 4 [ l ] / k 3 if k2 < k 4 [ l ] .We actually prefer a scheme in which the two nucleophiles compete for an intermediate which already has lost the leaving group and therefore use different symbols k B / k s in eq 1 to express the competition. In this case no assumption needs to be made about the relative magnitudes of kz and k , [ l ] .Highly instructive is a consideration of the case wherei.e., intermediate is trapped preferentially by 1. As eq 3 shows, the rate of disappearance of sulfite ion now is second order in sulfite ion.(3)By use of our value for k B / k s and the concentrations given in ref 3 calculations show that sulfonate ion 2 is the major product at high pH. But as the acidity is lowered and the ratio of thiamin base to sulfite ion concentrations increases the major product becomes bispyrimidine 4 as the reaction becomes second order in sulfite ion.1° Registry No. l.HCl,67-03-8; 5,77028-14-9; sulfite ion, 14265-45-3. S c h e m e I6 ' Me,Si 2 aq HCI quench * 2a, R = H; R = Me b, R = Me; R' = i-Bu 3a, R = H (93%) 4a, R = H (96%) b, R = Me (77%) b, R = Me (99%) '8 1 2 Jones o x o l y l o x d n chloride, Phd-'6 3 TiCIq,CH2C12, -3 0 -2 5 "C I I 5a, R = H (90%) b , R = M e ( 9 4 % ) la, R = H (82%) b, R = Me (84%) frameworks.2 In connection with ongoing synthetic efforts in these laboratories, we required an efficient entry to spiro[4.5]decadienones of general structure 1. The known and readily accessible dimedone derivatives 2 were seen as particularly attractive starting material^.^ We report herein a vinylsilane-mediated spiroannulation sequence which effects the desired overall transformation of the vinylogous esters 2 to the spirodienones 1 in excellent yield.4 R go* R ' O RyJ I I 2a. R = H: R' = Me l a , R = H b; R = Me; R = i-Bu b, R = Me Treatment of dimedone methyl ether (%a) with trans-P-(trimethylsilyl)vinyllithium5 (-78425 "C) followed by a quench with 5% aqueous HC1 pr...