Product (x) is :

Options

A.
B.
C.
D.

Answer

B

Explanation

Since there is no carbonyl group in the first product, the double bond of the exocyclic methylene group has reacted with methylenephosphorane. We may therefore propose a mechanism based on the assumption that the exocyclic double bond behaves like the double bond in a carbonyl group. The latter reaction is

However, in the case under consideration, there is no oxygen atom, phosphine and not phosphine oxide is eliminated, and a single and not a double bond is formed with the methylene group. This can be accommodated by replacing `O^(-)` by ` gt C^(-)` (of the five - membered ring), and then as follows:

In this sequence, we have proposed a complete parallel with the carbonyl group and this results in the formation of a spiro-compound containing phosphorus in one ring. The final product is also a spiro-compound, but since this contains a three-membered ring, it could be argued that this is less stable than its precursor – which contains a four - membered ring. This can be overcome by assuming that the three - membered ring is formed directly, the mechanism involving neighbouring group participation i.e.,

This route would be energetically more favourable than the other one and so is the more likely one.