Rearrangement cyclic alkynes

Cyclic alkynes (C9, Ci0, or Cn) also rearrange in the presence of bases to an equilibrium mixture containing cyclic allene [43]. The bases used were NaNH2-liq. NH3 at -33.4°C, KOH-C2H5OH at 131°-134°C (sealed tubes), KO-f-Bu-f-BuOH at 79.4°-l 20.0°C (sealed tubes) [44]. A solution of sodamide in liquid ammonia gave the most rapid ( to 3 hr) allene-acetylene interconversions of all systems examined. 

The Fritsch-Buttenberg-Wiechell rearrangement provides a method to expand a cyclic ketone to a cyclic acetylene with an additional carbon atom in the ring [34]. The ketone is first converted to a vinyl bromide or a vinylidene dibromide, from which the corresponding vinylidene carbene is produced with butyllithium. The vinylidene carbene then rearranges to a cyclic alkyne (Scheme 8-10). 

A characteristic feature of the reactions of cyclic alkynes is the tendency to release ring strain by changing the hybridization at the alkyne carbons from sp to sp. This is achieved either by rearrangement reactions or by intra- or intermolecular addition reactions. 

The first step in this scheme is a Michael addition of the nucleophile to the j5-carbon of the alkynyliodonium salt to give the ylide 102. Loss of iodobenzene from 102 gives alkylidenecarbene 103, which rearranges to alkyne 104 in the absence of external traps. This mechanism is experimentally supported by the isolation of cyclic by-products 108 besides the major products, the alkynyl esters 107 in the reaction of alkynyliodonium salt 105 with nucleophiles (equation 67). These cyclic enol ethers are the result of the insertion of the intermediate carbene 106 into the tertiary-8-carbon-hydrogen bond. 

Dipolar cycloaddition of pyridine N-oxide to the cyclic alkyne (68), followed by rearrangement, leads to the formation of the first reported 2//-azepine (69) this undergoes thermal rearrangement to the 2-substituted pyridine (70) (Scheme 28). ... 

The intramolecular Diels-Alder cycloadditions of 1-aminobenzofurans provide a simple synthesis of benzo[/r]quinolines. A-Acetyl-2-azetine undergoes facile 4-1-2-cycloaddition with cyclopentadiene, 5,5-dimethoxy-1,2,3,4-tetrachlorocyclopentadiene, hexachloropentadiene, and diphenylisobenzofuran to produce exclusively endo-cyc o-adducts. The highly strained cyclic alkyne bicyclo[2.2.1]hept-2-en-5-yne (108), prepared from the heptavalent iodine precursor (107), has been trapped using 1,3-diphenylisobenzofuran to yield the adduct (109) (Scheme 41). The reaction of sodium dinitroxytrioxide (110) with 1,3-diphenylisobenzofuran gives an initial 4-1-2-cycloadduct (111), which rearranges to the final product (112) (Scheme 42). " The... 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

The smooth allyloboration of alkynes is known to proceed via an allyl rearrangement, probably including a six-membered cyclic transition state. Thermal treatment of the product initiates a second allyloboration step and a vinyloboration thereafter the whole procedure [Eq. (37)] opens a synthesis of boraadamantanes by further reaction steps 76). 

Friedrichsen and co-workers (133) approached substituted benzotropolones from an aromatic substituted carbonyl ylide with a tethered alkyne as the intramolecular dipolarophUe (Scheme 4.67). Starting from an aromatic anhydride, Friedrichsen was able to make the tethered alkyne via addition of either pentyn-ol or hexyn-ol, then transform the recovered benzoic acid to the a-diazocarbonyl cycloaddition precursor. Addition of rhodium acetate resulted in the tandem formation of cyclic carbonyl ylide followed by cycloaddition of the tethered alkyne producing the tricyclic constrained ether 252. Addition of BF3 OEt2 opened the ether bridge, forming the benzotropylium ion, which subsequently rearranged to form the tricyclic benzotropolone (253). 

The reaction with 4-pentyn-l-ol gave only [Fe t/2-CH2=C(CH2)30) (CO)2(t/-C5H5)]+, and 3-hexyn-l-ol afforded (64, R = Et) (84) no evidence for the participation of the vinylidene tautomers was found. With ruthenium (45) and platinum (47) complexes, on the other hand, rearrangement to the vinylidene is faster than internal attack on the >/2-alkyne, and only the cyclic carbene complex is formed. 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

In contrast to the preceding mechanisms proposed for [3,3]-sigmatropic shifts, the mechanism of the silver-catalyzed oxy-Cope rearrangement was proposed as a stepwise process (Scheme 3.37). As usual, the reaction would be initiated by silver coordination to the alkyne moiety. Nucleophilic attack of this complex by the double bond would then lead to a cyclic cationic vinylsilver intermediate. Fragmentation would then give the dienone. 

Although not the best catalyst, AgSbF6 led to the rearranged product in 50% yield. This cascade reaction probably started with the well-known cyclization of the ketone to the alkyne on silver coordination, giving a cyclic oxonium intermediate that rearranged to furanone via an alkyl 1,2-migration (Scheme 3.54).82... 

The structure of 19 was unambiguously confirmed by an X-ray diffraction study. A mechanistic rationale is depicted in Scheme 11. After Co2(CO)6-complexed alkyne A is obtained, a Sn2 attack of the Co2(CO)6 fragment opens the epoxide moiety to afford intermediate B, which subsequently incorporates CO to give C. The latter rearranges into cobalt-stabilized cyclic allene species D. The net result is a [5 + 1] cyclization that creates the lactone group. Coordination of the tethered olefin leads to oxidative cyclization to give E. Finally, insertion of CO followed by reductive elimination affords the desired product 19. 

---