Spirocyclization, from phenolic

Fig. 31 Nachtsheim s dearomative carbo- and oxo-spirocyclizations from phenolic benzamido-acrylates...

Fig. 29 Ciufolini s synthesis of aza-spirocycles from para- and ort/io-phenolic sulfonamides and desymmetrization en route to the synthesis of a lepadiformine analogue...

In the same vein, Canesi s group also reported a method to prepare spiro[5.5] undecanyl ring systems 150 from phenols 147 para-tethered to a free alkyne moiety [98]. The phenoxenium ions 148 undergo carbo-spirocyclization into the vinyl cations 149, which are then trapped by solvent-derived nucleophiles to furnish the spirobicycles 150 in good yields (Fig. 37) [98]. 

The spirocyclic phosphazenes (138) resulting from the reaction with 2-azidoalcohols dimerize <82TL853>. In the adducts of phenols with a heterodiene function in the 2-position <77ZN(B)84l> the phosphorus atom also becomes the center of a tricyclic system (139) (Scheme 45). 

Chloranil oxidized a variety of methylarsenic(iii) esters, amides, or cyclic thioesters, products such as (89), for example, being obtained from a dithioester of methylarsonous acid. ° The analogous reaction with 2-iodo-l,3,2-dihetero-arsolans, however, proceeded with simultaneous cleavage of the As—I bond and formation of the spirocyclic arsenic(v) compound (90). The six-co-ordinate arsenic(v) anions (91) were produced by addition of an alcohol or phenol to an arsolan in the presence of triethylamine, the latter abstracting a proton and forming the required cation. ... 

The oxidative dearomatization of appropriately substituted phenolic substrates resulting in intramolecular cyclization with the formation of spirocyclic products represents one of the most powerful synthetic tools in modern organic synthesis (Seetion 3.1.11). Kita and coworkers were the first to report a catalytic variant of the oxidative spirocyclization reaction based on the in situ regeneration of a [bis(trifluoroacetoxy)iodo]arene from iodoarene using mCPBA as a terminal oxidant [2]. In a representative example, the oxidation of... 

Monomer Synthesis. Sila[l]ferrocenophane monomers such as (43) are readily available on a substantial laboratory scale (>100 g) from the reaction of dilithioferrocene tetramethylethylenediamine (fcLi2-TMEDA) with the appropriate dichloroorganosilane (166). Spirocyclic sila[l]ferrocenophanes such as (45) and (46) are also easily synthesized (167). Sila[l]ferrocenophane monomers with alkoxy, aryloxy, and amino substituents at silicon are readily accessible through reaction of dichlorosila[l]ferrocenophane with the appropriate alcohol, phenol or amine in the presence of base (168). 

Several other mechanistically distinct metal-catalyzed dearomatization procedures have been reported, and almost all involve phenol or naphthol derivatives undergoing dearomatization via intramolecular transformations. Intramolecular Pd- and Rh-catalyzed C4-arylation and alkylation of /)ara-substituted phenols has been used to construct compounds of general structure 82 (Fig. 15.1) [86]. These reactions rely on generation of electrophilic aryl or alkyl o-metal complex intermediates that participate in tandem C4 metalation-reductive elimination with an attached phenol. Ruthenium- and Pt-catalyzed reactions of naphthalenes and alkynes deliver spirocyclic products such as 83 [87, 88]. An asymmetric intramolecular naphthalene dearomatization catalyzed by Pd(0)-phosphine complexes has been used to prepare carbazole derivatives 84 in good enantiomeric excess from l-(AI-2-bromophenyl)aminonaphthalene precursors [89]. 

For their 2010 synthesis of the immunosuppressive polyketides dalesconols A and B [143], Snyder and co-workers developed an outstanding one-pot cascade to forge the bicyclo[5.3.0]decanyl core of these targets from the judiciously functionalized and protected precursors 276. The ultimate addition of DIB in TFE at low temperature served to transform in situ the phenols 277 into the five-membered carbo-spirocyclic cyclohexa-2,5-dienone intermediates 278, which were then efficiently converted into dalesconols A and B (Fig. 67) [143]. 

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