Chroman ring closure with

Chroman ring closure with halogenation 19, 320 Chromatography, preparative 19, 944... 

The formation of chromane derivatives has also been realised in the palladium catalyzed intramolecular nucleophilic substitution of allyl carbonates (Tsuji-Trost reaction). In most cases the reaction is accompanied by the formation of a new centre of chirality. Using Trost s chiral ligand the ring closure was carried out in an enantioselective manner. The asymmetric allylation of the phenol derivative shown in 4.20. was achieved both in good yield and with excellent selectivity.23... 

Ring closure of a 2-methoxyphenyl derivative of propanol to a chroman has also been achieved. Treatment of the aryloxazoline (259) with sodium hydride yielded the chroman (260) (81JOC783). The intramolecular nucleophilic displacement of the o-methoxy group is promoted through oxazoline activation and proceeds through an addition-elimination sequence. The initial attack involves coordination of the metal alkoxide to both the oxazoline moiety and the methoxy group, and aromatization follows with displacement of methoxide ion (Scheme 67). Hydrolysis of the oxazoline moiety to a carboxyl group has been accomplished. 

Glycoside formation followed by ring closure occurred with formation of a chroman in the reaction of the 3,4,6-tri-0-benzyl-2-C-acetoxymethylgalactal shown with 4-methylphenol in dichloromethane containing boron trifluoride-etherate at 0°C to afford the a-product in 70% yield by way of the C-2-methylene-O-glucoside which could be prepared separately and converted to the same pyranobenzopyran under similar conditions (ref. 38). 

The cyclization of an aryl radical is a useful process for the preparation of (hetero)cyclic compounds such as tetrahydrobenzofurans, chromanes, indanes, indolines and tetrahydroquinolines [147]. The synthetic strategy involves the generation of an aryl radical which subsequently adds to an unsaturated moiety, most usually in a 5-exo or a 6-exo trig process (Scheme 13.19). After ring closure, the intermediate cyclized radical 93 can either be reduced (pathway A) or it can react further with a nucleophile (SRNl-type process, pathway C) or with another radical acceptor to yield, after reduction, compounds of type 94 (pathway B). 

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