Treatment of acetylenic or olefinic carbonates and esters with a low-valent titanium reagent diisopropoxy(eta(2)-propene)titanium (1), readily generated by the reaction of Ti(O-i-Pr)(4) or ClTi(O-i-Pr)(3) with 2i-PrMgX, resulted in an intramolecular nucleophilic acyl substitution (INAS) reaction to afford organotitanium compounds having a carbonyl functional group, in good to excellent yields. Thus, the treatment of alkyl alkynyl carbonates 2 or alkyl alkenyl carbonates 4 with 1 gave organotitanium compounds having a lactone and/or ester group. Similarly, alkyl alkynoates 10 or alkynyl esters 14 of carboxylic acids reacted with 1 to give organotitanium compounds having a cyclic or acyclic ketone group, respectively. Thus, the reaction provides, after hydrolysis, five- or six-membered alpha-alkylidene lactones and/or alpha,beta-unsaturated esters from 2, gamma-butyrolactone derivatives from 4, five- or six-membered alpha-alkylidene cyclic ketones from 10, and acyclic alpha,beta-unsaturated ketones 15 from 14. In all cases, the yields are excellent and the generation of the organotitanium compounds was confirmed by deuterolysis. The organotitaniums 6 and 11c reacted smoothly with iodine to afford 2-(iodomethyl)-4-butanolide (9) and alpha-[iodo(trimethylsilyl)methylidene]cyclopentanone. respectively. The organotitanium compounds obtained here also reacted with aldehydes to give the corresponding adducts, thus opening up a new access to substituted alpha,beta-butenolides from 2, to gamma-butyrolactones from 4. and to the corresponding tetrasubstituted furan from 10 and 14.