Allene amides, cyclization

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

Propargylsilanes show predictable behavior as cyclization terminators. Internal additions of propy-nylsilanes to acyclic -acyliminium ions promoted by Lewis acids or protic acids yield a-allenic amides or carbamates (Scheme 54). ... 

Optically active allenes have been extensively studied by Tius for the asymmetric formation of the corresponding Nazarov cyclization products (Scheme 16.36) [132, 133], As an illustrative example, cyclization of enone 264 furnished cyclopentenone 265 with a high degree of stereoinduction (95% ee) [132], The intermediate cyclization precursor 264 had been prepared by addition of vinyllithium 263 to optically active allene amide 262 (98% ee). 

A ring opening reaction of (1-lactams promoted by methoxide generated nitrogen nucleophiles in situ that subsequently added to proximal allenes producing trisubstituted pyrroles <06CC2616>. In the event, treatment of (3-lactam 3 with MeONa led to pyrrole-2-acetic ester 4 after cleavage of the amide bond, 5-exo-dig cyclization, and loss of methanol. The sequence was notable as no metal catalyst was required. 

The palladium(0)-catalyzed cyclization of amide-allenes via a carbopalladation has been developed by several groups. The reaction proceeds through the carbopalladation of the allene moiety with an organopalladium species (R-Pd-X), generated by oxidative addition of R-X to palladium(O), and subsequent reductive elimination of the resultant 7r-allylpalladium intermediate.47,47a 47f... 

The addition of allenyl ether-derived anions to Weinreb [4] or to morpholino amides [5] follows a slightly different pathway (Eq. 13.2). For example, the addition of lithioallene 6 to Weinreb amide 7 at -78 °C, followed by quenching the reaction with aqueous NaH2P04 and allowing the mixture to warm to room temperature leads to cyclopentenone 9 in 80% yield [6]. The presumed intermediate of this reaction, allenyl vinyl ketone 8, was not isolated, as it underwent cyclization to 9 spontaneously [7]. These are exceptionally mild conditions for a Nazarov reaction and are probably a reflection of the strain that is present in the allene function, and also the low barrier for approach of the sp and sp2 carbon atoms. What is also noteworthy is the marked kinetic preference for the formation of the Z-isomer of the exocyclic double bond in 9. Had the Nazarov cyclization of 8 been conducted with catalysis by strong acid, it is unlikely that the kinetic product would have been observed. 

Aluminum—tetradentate ligand catalyst system, in epoxide homopolymerization, 11, 601 Aluminum(I) tetrahedra, synthesis, 9, 262 Aluminum(III)-tin exchange, process, 9, 265 Aluminum-transition metal bonds, characteristics, 9, 264 Amavadine, for alkane carboxylations, 10, 234—235 Ambruticin S, via ring-closing diene metathesis, 11, 218 Amide-allenes, cyclizations, 10, 718 Amide ether complexes, with Zr(IV) and Hf(IV), 4, 783 Amide hybrid ligands, in organometallic synthesis, 1, 64 Amides... 

The corresponding nitrogen-containing heterocycles are accessible by silver-catalyzed cyclization of allenic amines, amides, or oximes.334 Thus, Dieter and Yu335 have reported the efficient transformation of various a-aminoallenes 397 or 399 into mono- or bicyclic 3-pyrrolines 398 or 400 in the presence of AgN03 (Scheme 116). Unfortunately, a rather high catalyst loading was required. 

When the chiral allenic amine 1 was cyclized using silver tetrafluoroborate, transfer of chirality occurred from the allenic moiety to the C-2 stereogenic center, as determined by conversion of 2 into the diastcrcomeric mixture of the (+)-(5)-0-methylmandelic amides of the debenzylated product. The high asymmetric induction was ascribed to the formation of a dissymmetric silver-allene complex229. 

The rhodium-cataiyzed intramoiecuiar [5+2] cycioaddition of an allene and vinylcyclopropane was the key step in the asymmetric total synthesis of the trinorguaiane sesquiterpene (+)-dictamnol by P.A. Wender and co-workers.The cyclization precursor allene-cyclopropane was assembled starting from commercially available cyclopropane-carbaldehyde. Using the HWE oiefination, the Weinreb s amide moiety was installed and subsequently reacted with a primary alkyllithium that was generated via lithium-halogen exchange. 

The allene PrCH=C=CHCH2NH2 isomerizes to the 3-pyrroline (122) under the influence of silver tetrafluoroborate. Photolysis of aroyl azides in the presence of diketen (123) yields the hydroxy-pyrrolinones (124).AT-Ray analysis has shown that the adduct of the imine Pr"CH=NPr to N-phenylmaleimide has structure (125). Treatment of dichloromaleimide with ethoxycarbonyl-methylenetriphenylphosphorane affords the Wittig product (126). The formation of the pyrrolidinone (128) in the thermolysis of the iV-cyclohexenylacryl-amide (127) represents an intramolecular ene-reaction (see arrows). The perfluoropyrrolidinone (130) results from the reaction of the cyclobutane (129) with potassium fluoride. Pyrrolidinols (131) are obtained in moderate yields by photochemical cyclization of the amides ArCOCH2CH2NBzCH2Ph. ... 

Other cyclizations of allenols have been realized by means of gold-catalysts [186, 187], gold and platinum catalysts [188], and with lanthanum amide catalysts [189]. Intermolecular additions of alcohols to allenes were also catalyzed by cationic gold(I) complexes with carbene [190] or phosphane spectator ligands... 

More recently, the Brummond laboratory reported a series of Rh-catalyzed allenic Alder-ene reactions affording 5- and e-lactams 89 from amide-tethered alleneynes 88 (Scheme 46) (94). Higher temperature was required for the reaction to proceed efficiently. This observation could be attributed to the secondary amide, because the reaction of protected amides proceeded at room temperature to afford cyclized product in less than 1 h. The reaction is somewhat sensitive to terminal alkyne in that product yields for amide-tethered allenyne substrates with terminal alkyne were lower. 

The reaction of dithio-esters (19) with bromoalkynes in the presence of potassium amide in liquid ammonia yields the keten dithioacetal (20), which upon heating undergoes an electrocyclic rearrangement to the allenic dithio-esters (21). Acid-catalysed or strongly base-catalysed cyclization yields the thiophen derivatives (22), whereas weak bases yield the 2H-thiopyran (23). 

---