Pergamon Tetrahedron Letters, Vol. 38, No. 49, pp. 8433-8434, 1997 © 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain PII: S0040-4039(97)10272-6 0040-4039/97 $17.00 + 0.00 BEYOND THE MEDIUM RING: A [4 + 4] CYCLOADDITION/FRAGMENTATION SYNTHESIS OF ELEVEN-MEMBERED RINGS Scott McN. Sieburth,* Taleb H. AI-Tel, and David Rucando Department of Chemistry, State University of New York at Stony Brook Stony Brook, New York 11794-3400 Abstract: Reduction of the polycyclic product of intramolecular 2-pyridone photocycloaddition with lithium/ammonia yields a functionalized cycloundecane, via an apparent multi-step reaction sequence. © 1997 Elsevier Science Ltd. The coupling of cycloaddition and fragmentation reactions is a venerable approach to the less accessible ring sizes, particularly seven and eight-membered carbocycles; ] the evolving methods for medium ring construction via higher order cycloadditions 2 therefore sets the stage for even larger rings. We report here the serendipitous preparation of an eleven-membered ring by fragmentation of an 8-5 ring system. hv/A H2 76% PO-C HN,,,,,~O 0 ref. 3 95% "~ 1 2 3 5 Scheme I. Photo-[4 + 4]-cycloaddition and lithium-ammonia reduction leads to cycloundecanc product 5. Bis-2-pyridone | undergoes an efficient [4 + 4] photocycloaddition to yield the trans product 2 along with its cis isomer. Isomerization of the more labile cis isomer through a simple thermal and photochemical procedure results in the isolation of 2 in 76% yield) During a study of the chemistry of photoproduct 2, with a goal of opening the amide bonds, the alkenes were hydrogenated and the resulting product 3 was subjected to lithium-ammonia reduction. These o2 conditions have been used tO reduce amides to l!i~ ct~ //~ aminals 4 (e.g., 4), an intermediate for s reduction of the carbonyls to methyl groups. ~ cts In addition to a compound tentatively (:2 ~ ~ "~//-m'mm m identified a s 4 , another product could be ~ m m ~ ( ~ ) / . ~ isolated in an unoptimized yield of 25%. This m product retained both amide groups but one (:3 was now a secondary amide, with a well defined amide proton coupled to an N-methyl m3 group. A number of possible structures were c7 considered, however X-ray crystallography unequivocally identified the product as 5. ol The X-ray structure of 55 shows that Figure 1, X-ray structure of 5. 8433