Novel synthesis of alpha-PNA monomers by U-4CR.

A novel synthesis of alpha-PNA monomers was carried out by U-4CR, followed by photochemical cleavage of the 2-nitrobenzyl group and selective hydrolysis in the presence of 10% HCl in THF. Three of four functional components in the U-4CR were specially protected: cyclohexenyl isocyanide, Boc for protecting the amino group of glycine, and 2-nitrobenzyl group as a photocage (photoremovable protecting group) for ammonia. The amino group of aldehyde-containing adenine is too weak to interfere with the U-4CR, so that it is not necessary to be protected.