Prenyl bromide, reaction with phenols

An important example is the reaction of prenyl bromide with phenols. This is simply carried out with K2CO3 in acetone as phenols are acidic enough (pKa 10) to be substantially deprotonated by carbonate. The product is essentially entirely from the S>f2 route, and is used in the Claisen rearrangement (Chapter 36). 

In the second approach (B), because of a low initial yield in the first method, the acid-catalysed reaction of the phenol in dioxane containing boron trifluoride etherate with the prenyl alcohol was used. In this way 2-geranyl, 2-farnesyl, 2-phytyl and 2-nonaprenylphenol were obtained in unstated yields, accompanied by the 4-isomer in each case. In the third strategy (C) the prenyl bromide was... 

A combinatorial natural-product-like and diversity-orientated library of 10 000 benzopyran-based small molecules was constructed by Nicolaou and coworkers [74, 75]. They chose a solid-phase approach and an anchoring strategy that does not limit complexity building operations (Scheme 14.11). They used a polystyrene-based selenyl bromide resin (43) on which substrates can be immobilized by electrophilic cyclization reactions. Here, ortho-prenylated phenol 44 was reacted with the selenyl bromide (43) to form the benzopyran scaffold (45) via a 6-endo-trig cyclization. 

Although the dipotassium salt of hydroquinone in refluxing xylene with 3-methylbut-2-enyl bromide (ref. 10) similarly gave 4-hydroxy-2-(3-methylbuten-2-yl)phenol in 50% yield, reactions were not described for the dipotassium salt of resorcinol with a prenyl chloride. The potassium salt of 3-methoxyphenol with allyl chloride gave 2-allyl-3-methoxyphenol in 42% yield. 

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