Intramolecular reactions asymmetric annulation

In addition to rhodium-catalyzed [5+n]-type annulation of VCPs, [3+ ] annulation reactions of VCPs have also been developed. Asymmetric rhodium(I)-catalyzed intramolecular [3+2] annulation of 1-alkynyl-l-vinylcyclopropanes gave bicyclo [3.3.0] compounds with an all-carbon chiral quaternary stereocenter... 

Another key event in the history of organocatalytic reaction was the discovery of efficient r-proline-mediated asymmetric Robinson annulation reported during the early 1970s. The so-called Hajos-Parrish-Eder-Sauer-Wiechert reaction (an intramolecular aldol reaction) allowed access to some of the key intermediates for the synthesis of natural products (Scheme 1.4) [37, 38], and offered a practical and enantioselective route to the Wieland-Miescher ketone [39]. It is pertinent to note, that this chemistry is rooted in the early studies of Langenbeck and in the extensive investigations work of Stork and co-workers on enamine chemistry... 

In addition to cationic cyclizations, other conditions for the cyclization of polyenes and of ene-ynes to steroids have been investigated. Oxidative free-radical cyclizations of polyenes produce steroid nuclei with exquisite stereocontrol. For example, treatment of (259) and (260) with Mn(III) and Cu(II) afford the D-homo-5a-androstane-3-ones (261) and (262), respectively, in approximately 30% yield. In this cyclization, seven asymmetric centers are established in one chemical step (226,227). Another intramolecular cyclization reaction of iodo-ene poly-ynes was reported using a carbopaUadation cascade terminated by carbonylation. This carbometalation—carbonylation cascade using CO at 111 kPa (1.1 atm) at 70°C converted an acycHc iodo—tetra-yne (263) to a D-homo-steroid nucleus (264) [162878-44-6] in approximately 80% yield in one chemical step (228). Intramolecular aimulations between two alkynes and a chromium or tungsten carbene complex have been examined for the formation of a variety of different fiised-ring systems. A tandem Diels-Alder—two-alkyne annulation of a triynylcarbene complex demonstrated the feasibiHty of this strategy for the synthesis of steroid nuclei. Complex (265) was prepared in two steps from commercially available materials. Treatment of (265) with Danishefsky s diene in CH CN at room temperature under an atmosphere of carbon monoxide (101.3 kPa = 1 atm), followed by heating the reaction mixture to 110°C, provided (266) in 62% yield (TBS = tert — butyldimethylsilyl). In a second experiment, a sequential Diels-Alder—two-alkyne annulation of triynylcarbene complex (267) afforded a nonaromatic steroid nucleus (269) in approximately 50% overall yield from the acycHc precursors (229). 

In the laboratory of J.D. White, the asymmetric total synthesis of (+)-codeine was accomplished. The Robinson annulation was the method of choice to build a phenanthrenone precursor starting from a substituted tetralone derivative. As it is usuaiiy the case, the isolation of the Michael adduct allowed the intramolecular aldol reaction to proceed cleanly and to afford a higher yield of the annulated product. 

Anilide 2a catalysed asymmetric intramolecular Michael reaction of formyl enones to chiral cyclic keto-aldehydes in excellent yields with good stereoselectivity (eqn. (1) in Scheme 6.3). The intramolecular Michael addition of a ketosulfone to an unsaturated ketone (eqn. (2) in Scheme 6.3) catalysed by 15e has heen used as a key step in the synthesis of the carbon tricyclic framework of Lycopodine. The same sulfonylprolinamide served as catalyst in the construction of all-carhon substituted quaternary stereocentre via Robinson-type annulation process (eqn. (3) in Scheme 6.3). 

More recently, Enders et al. disclosed a facile access to tetracyclic double annulated indole derivatives 175, which basically relies on the chemistry of the acidic 2-substituted indole and its nitrogen nucleophilicity. Indeed, the employed quadruple cascade is initiated by the asymmetric aza-Michael-type A-alkylation of indole-2-methylene malono-nitrile derivative 174 to o,p-unsaturated aldehydes 95 under iminium activation (Scheme 2.57). The next weU-known enamine-iminium-enamine sequence, which practically is realized with an intramolecular Michael addition followed by a further intermolecular Michael and aldol reactions, gives access to the titled tetracyclic indole scaffold 175 with A-fused 5-membered rings annulated to cyclohexanes in both diastereo- and enantioselectivity [83]. 

In this work, the combination of cinchona-alkaloid-derived primary amine and Au(I) phosphine complex has been employed to effect two consecutive Friedel-Crafts-type reactions of pyrrole in one pot to furnish 2,3-annulated pyrroles containing a seven-membered ring (eq 1) This reaction proceeds through initial asymmetric 1,4-addition of p)rrole to enones catalyzed by the primary amine followed by an intramolecular 7-c/ido-dig cycliza-tion catalyzed by the Au(I) phosphine complex. 

The intramolecular benzoin condensation between an aldehyde and ketone was virtually unknown until 2003, when Suzuki and co-workers established that thiazolylidine catalysts could effect this transformation in good to excellent yield for the synthesis of functionalized preanthraquinones such as 30. Subsequent to this work, reports of annulation reactions of a variety of ketoaldehyde substrates, as well as asymmetric variants, have appeared in the literature. ... 

In contrast to the limited success in achieving direct asymmetric intermolecular Stetter reactions, there have been an abundance of reports detailing highly enantioselective intramolecular Stetter annulations. The first reported example was published by Enders and co-workers in 1996, in which the Ciganek reaction was rendered modestly asymmetric through the use of... 

The introduction of the activated allylic bromides and Morita-Baylis-HiUman acetates and carbonates pioneered the development of a number of phosphine-catalyzed reactions in subsequent years [45]. Interestingly, the asymmetric variant of this type of transformation only appeared in the literature seven years later. In 2010, Tang, Zhou, and coworkers disclosed a highly enantioselective intramolecular ylide [3-1-2] annulation using spirobiindane-based phosphine catalyst 31 (Scheme 20.27). BINAP was found inactive in this reaction even at an elevated temperature (70°C). Notably, both optically active benzobicyclo[4.3.0] compounds 32 and 32 with three continuous stereogenic centers could be obtained as major products in high yields and stereoselectivities just by a choice of an additive [Ti(OPr )4], which can block the isomerization of the double bond [46]. 

Robinson Annulations Asilicagel-absorbedaminoacidsalt(39)-catalyzed asymmetric intramolecular Robinson annulation reaction with 38 was developed (Scheme 1.12). A tricyclic ring stracture 40 was obtained in 84% yield and up to 97% ee [15]. hitermolecular Robinson annulations with structurally diverse aldehydes and unsaturated ketones were also developed [16]. 

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