Beaudry et al. in Chemical Reviews, 2005, Vol. 105, No. 12 (attached below) write:

The biosyntheses of vitamin D3 (76a) and vitamin
D 2 (76b) are arguably the best-known examples of
electrocyclic reactions in Nature (Scheme 18). The
transformations of 7-dehydrocholesterol (74a) and
ergosterol (74b) into vitamin D 3 (76a) and vitamin
D 2 (76b), respectively, have been extensively re-
viewed, and they are therefore only briefly sum-
marized here.35 Photochemical conrotatory ring open-
ing of 74a leads to pre-vitamin D3 (75a).

However, they cite only structures and not the frontier MO analysis desired. I am unable to find the “extensive reviews” they are talking about. There is the book section “Photochemistry of vitamin D and its isomers and of simple trienes” apparently at https://onlinelibrary.wiley.com/doi/book/10.1002/9780470133415#page=311, but it’s from a 1979 book that I can’t seem to find database access for. I realize the solution must be “too trivial to publish” in research articles but I wonder why I can’t find the MO solution in most modern textbooks if it is the “best-studied” reaction. If the MO solution is so trivial, why don’t we encounter it earlier in our biochemical education?