46(P01) α-(トリメチルシリルメチル)ジビニルケトンのNazarov反応を用いるスピロ炭素骨格の合成(ポスター発表の部)
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α-Trimethylsilylmethyl-α,β-unsaturated ester, or its equivalent, is a unique block, since this moiety can react formally as both a nucleophile and an electrophile at the same carbon, which makes it easy to synthesize odd-membered cycles. α -(Trimethylsilylmethyl)divinyl ketone has the same structural feature and must be a good substrate for the Nazarov cyclization reaction. Here we report a synthesis of spiro[4.5]decane ring system, using Nazarov cyclization of α-(trimethylsil ylmethyl)divinyl ketone derivative, and some related reactions. The Nazarov cyclization of 1 was first carried out using FeCl_3 as a Lewis acid in CH_2Cl_2 at-60℃ to room temperature, giving 4a and 4b (ratio 5:1) in 59% yield. Compound 2 afforded 5a and 5b (61%, ratio 5:1), while 3 gave 6 (67%) (Scheme 1). The analogous compound 7 afforded 8 in 56% yield by treatment at 0℃ for 4h. While compound 9 was obtained in 62% yield when 7 was treated at room temperature for 24h. It was found that 9 is formed from 7 via 8 as the intermediate (Scheme 2). Compounds 10a and 10b gave 11 a,b at 0℃ (11a:11b=4:1 from 10a and 1:1 from 10b), and 12a-e at room temperature. The results of the tandem Nazarov cyclization-rearrangement reaction are summarized in Table 1. Substituent effect on divinyl ketone was studied using 13a,b and 15 as the substrates. Both 13a and 13b afforded 14 in good yields (93% from 13a and 91% from 13b), while in contrast, 15 afforded spiro compound having endocyclic double-bond 16 in 55% yield (Scheme 3). Similarly, 17 afforded 18 in 46% yield. Thus it was suggested that the position of the double bond in the product is determined by the presence/absence of α'-substituent. We propose the reaction mechanism as shown in Scheme 4. The effect of silicon atom was also studied. Compounds 19 and 20, desilylated substrate of 15 and 3, respectively, were treated under Nazarov condition. The same products, 16 and 6, were obtained but the yields were decreased (Scheme 5). These results indicates that the presence of trimethylsilyl group is essential in its yield, by stabilization of cationic intermediate. After the above results, compounds 21 a,b were prepared and the Nazarov cyclization reaction was examined, giving the expected product 22 in 73% and 61% yields, respectively.The stereoselectivity obtained from 10a can be explained by steric interaction between methyl group and C=C double bond as shown in Scheme 6. The stereoselectivity from 1 and 2 can be rationalized by steric interaction between cyclohexane ring and silicon-bearing carbon (Scheme 6). The substrates used in this study were synthesized as described in Scheme 7.
- 天然有機化合物討論会の論文
- 1999-09-01
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