Studies On The Synthesis And Reactions Of Allenes

Allenes can undergo many reactions,such as electrophilic addition,nucleophilic addition,free radical addition reaction,etc.Under the catalysis of transition metals,coupling cyclization reactions could also be achieved.What's more,allenes have axial chirality,which is different from alkenes and alkynes.The axial chirality could be transferred to central chirality under suitable reaction conditions,which demands the highly enantioselective synthesis of allenes.The main focus of my dissertation is on the studies of synthesis and reactions of allenes.The dissertation includes three parts:Part I:Studies on the N-heterocyclic carbene-catalyzed reactions of electron-deficient allenesN-Heterocyclic carbene-catalyzed reactions of simple aldehydes,a,p-unsaturated enals,and a,?-unsaturated ynals have been well studied.However,such catalyzed reactions of electron-deficient allenes have not been carefully explored yet.1.We have developed a N-heterocyclic carbene?NHC?-promoted annulation reaction of allenals with chalcones,affording polysubstituted pyranyl aldehydes.Three mechanistic possible pathways have been proposed.The NHC attacks the center carbon of allenals to form a delocalized anion intermediate or acts as a base for deprotonation.These intermediates then undergo a 1,4-addition with chalcones,followed by sequential reactions to give the final products.2.A general N-heterocyclic carbene-catalyzed reaction of readily available aromatic aldehydes and 1,2-allenones to cyclopent-2-enone-4-ols with a high diastereoselectivity has been developed.Mechanistic studies showed that there are four possible pathways.3.?E?-4-Oxo-2-butenoates have been prepared highly stereoselectively from readily available aldehydes and allenoates catalyzed by N-heterocyclic carbene.Mechanistic studies showed that chloroform acted as proton shuttle.The reactivity of the intermediate was affected by the amount of water added deliberately,which finally decided the stereoselectivity of the product.Part ?:Studies on the highly enantioselective synthesis of 1,3-disubstituted allenols from aldehydes,terminal alkynols,and optically active dimethylprolinol catalyzed by CuBr₂We have developed a general CuBr₂-catalyzed method for the synthesis of optically active 1,3-disubstituted allenols through a three-component coupling of aldehydes,terminal alkynols,and optically active dimethylprolinol based on our previous protocol.Both aliphatic and aromatic aldehydes were compatible.Furthermore,we found that the enantioselectivity of the products was not affected by the longth of the carbon chains between alcohol functionality and allene skeleton.?S?-Dimethylprolinol gave better ee than?S?-diphenylprolinol in most cases.These optically active 1,3-disubstituted allenols have wide application in organic transformation.We also realized the total synthesis of phlomic acid.Part ?:Studies on the gram scale synthesis of some naturally occurring 2,7-dioxygenated carbazole alkaloidsWe have synthesized some naturally occurring 2,7-dioxygenated carbazole alkaloids,including 7-methoxy-O-methylmukonal,clausine-O,clausine-K?clauszoline-J?,clausine-H?clauszoline-C?,7-methoxymukonal,and carbazole compound methyl 2-hydroxy-7-methoxy-9H-carbazole-3-carboxylate on the gram scale based on the efficient Au-catalyzed cyclization reaction of readily available 1-benzyl-6-methoxy-1H-indole-2-carbaldehyde and 1-methoxypropa-1,2-diene.Among them,7-methoxymukonal and methyl 2-hydroxy-7-methoxy-9H-carbazole-3-carboxylate were synthesized for the first time.What's more,the structure of methyl 2-hydroxy-7-methoxy-9H-carbazole-3-carboxylate we prepared was further confirmed by X-ray single crystal diffraction study via its ethylation reaction,which indicated that clausine-TY reported by Taufiq-Yap et al.in 2007 should be reassigned.