Synthesis of spirohydantoins and spiro-2,5-diketopiperazines via resin-bound cyclic alpha,alpha-disubstituted alpha-amino esters

A seven-step solid-phase synthesis of spirohydantoins and an eight-step solid-phase synthesis of spiro-2,5-diketopiperazines is reported. Key intermediate in the synthesis of both compound libraries is the resin-bound cyclic alpha,alpha-disubstituted alpha-amino ester, which can be obtained after selective homogeneous reduction of the aliphatic nitro ester using tin(II) chloride dihydrate. Nitro ester, in turn, is synthesized by a high-pressure-assisted [4 + 2] cycloaddition of resin-bound nitro alkene and butadiene, whereas nitro alkene is obtained by a Knoevenagel condensation of resin-bound nitro acetate with an imine. Novel spirohydantoins are obtained by isocyanate coupling with the resin-bound amino ester 5, followed by cyclization cleavage using a base. Novel spiro-2,5-diketopiperazines are obtained by PyBOP coupling of a Fmoc-protected amino acid with resin-bound amino ester, followed by Fmoc deprotection and an acid-assisted cyclization cleavage. After preparation of seven different resin-bound alpha,alpha-disubstituted alpha-amino esters, a 7 x 8 compound library of spirohydantoins was synthesized using eight different isocyanates, and a 7 x 8 compound library of spiro-2,5-diketopiperazines was synthesized using eight different Fmoc amino acids.