Several mechanistic possibilities were considered. The first step is undoubtedly attack of the organolithium species 1 on the carbonyl group of 2 to produce 4, which has two obvious pathways available to it. A concerted, anion-accelerated [1, 3]-sigmatropic shift5 could occur, affording 5 directly. A second possibility is ring opening of 4 to form intermediates 6 or 7, which can cyclize to 5 via a Michael addition or electrocyclic ring closure, respectively ( ...
![7-bromo-1,4-dithiaspiro[4.5]dec-6-ene Structure](https://image.chemsrc.com/caspic/197/81770-76-5.png)
![Lithium, 1,4-dithiaspiro[4.5]dec-6-en-7-yl Structure](https://image.chemsrc.com/caspic/112/89279-01-6.png)
![6-bromo-1,4-dioxaspiro[4.5]dec-6-ene Structure](https://image.chemsrc.com/caspic/094/70156-98-8.png)
![Lithium, 1,4-dioxaspiro[4.5]dec-6-en-6-yl Structure](https://image.chemsrc.com/caspic/443/89279-10-7.png)
