Q4(b). Photochemical [1,3]-H shift; Diels–Alder of 1-methoxycyclopentadiene

Introduction. (i) is a [1,3]-sigmatropic hydrogen shift: an allylic hydrogen moves to the other end of the allyl system. (ii) is a Diels–Alder reaction with an unsymmetrical diene and dienophile, so both regiochemistry and endo stereochemistry have to be shown.

(i) Photochemical [1,3]-H shift of isopropylidenecyclopentane

For the FMO analysis the transition state is treated as an H atom (1s orbital) moving across an allyl radical. In the ground state the allyl SOMO is ψ₂, whose terminal lobes have opposite phases on the same face: a suprafacial [1,3] shift is forbidden thermally (an antarafacial one is geometrically impossible over three carbons). On irradiation an electron is promoted to ψ₃, whose terminal lobes have the same phase on the same face. The 1s orbital can now bond to both termini on one face: the suprafacial [1,3]-H shift is photochemically allowed.

The allylic hydrogens of the substrate lie on two kinds of carbon: the ring CH₂ next to the exocyclic C=C, and the two methyl groups. A [1,3] shift from the ring moves the double bond into the ring; a shift from a methyl group moves it out to the isopropenyl group.

Figure: Suprafacial [1,3]-H shift: H bonds in phase to both termini on the top face.

Scheme: Products of the two [1,3]-H shifts.

Products: 1-(propan-2-yl)cyclopent-1-ene (H from a ring CH₂; the endocyclic trisubstituted alkene, major) and (prop-1-en-2-yl)cyclopentane (H from a methyl group).

(ii) 1-Methoxycyclopenta-1,3-diene and but-3-en-2-one

• Regiochemistry (“ortho rule”). The methoxy group at a diene terminus (C-1) raises the HOMO and puts its largest coefficient at the far terminus, C-4. The enone LUMO has its largest coefficient at the β-carbon (CH₂=). These two large lobes bond first, so the acetyl-bearing carbon bonds to C-1: in the product OMe and COCH₃ sit on adjacent carbons (1,2-, “ortho”).
• Stereochemistry (Alder endo rule). In the endo transition state the C=O of the acetyl group lies under the diene, where its π orbital overlaps C-2 and C-3 (secondary orbital interaction). The kinetic product has the acetyl group endo, on the face of the bicycle that carries the C=C bridge, anti to the CH₂ bridge.

Major product: endo-1-(1-methoxybicyclo[2.2.1]hept-5-en-2-yl)ethan-1-one, formed as the racemate. Here the C5=C6 bond (from the diene) is at the back, and the acetyl group points down and away from the CH₂ bridge (C-7), that is, endo.

Figure: Endo adduct (racemic): OMe on the bridgehead C-1, acetyl on C-2.

Conclusion. (i) Light allows a suprafacial [1,3]-H shift (ψ₃ of the allyl part has in-phase termini), giving mainly 1-isopropylcyclopentene with some isopropenylcyclopentane. (ii) The Diels–Alder adduct has OMe and acetyl on adjacent carbons (ortho rule) and the acetyl endo.

Application. Regio- and endo-selective Diels–Alder reactions of substituted cyclopentadienes were the key steps in Corey's prostaglandin synthesis; photochemical H shifts are part of the vitamin D photochemistry in skin.