Selective O-Deallylation of o-Allyloxyanisoles

Abstract

o-Allyloxyanisoles are selectively and easily deallylated by treatment with sec- or tert-butyllithium at low temperature. The reaction is proposed to proceed through a tandem intermolecular carbolithiation-β-elimination process, which can also be considered an S_N2′ attack of the organolithium on the allyl ether.

Key words

protecting groups - lithium - eliminations - phenols - ethers

References and Notes

• 1 Kocienski PJ. Protective Groups   3rd ed.:  Thieme; Stuttgart: 2003. 
• For O-deallylation procedures, see:
• 2a Ito H. Taguchi T. Hanzawa Y. J. Org. Chem.  1993,  58:  774 
• 2b Guibe F. Tetrahedron  1997,  53:  13509 
• 2c Taniguchi T. Ogasawara K. Angew. Chem. Int. Ed.  1998,  37:  1136 
• 2d Thomas RM. Mohan GH. Iyengar DS. Tetrahedron Lett.  1997,  38:  4721 
• 2e Thomas RM. Reddy GS. Iyengar DS. Tetrahedron Lett.  1999,  40:  7293 
• 2f Tsukamoto H. Kondo Y. Synlett  2003,  1061 
• 3 Barluenga J. Fañanás FJ. Sanz R. Marcos C. Ignacio JM. Chem. Commun.  2005,  933 
• 4 Broaddus CD. J. Org. Chem.  1965,  30:  4131 
• 5 Bailey WF. England MD. Mealy MJ. Thongsornkleeb C. Teng L. Org. Lett.  2002,  4:  489 
• 6a Barluenga J. Fañanás FJ. Sanz R. Marcos C. Trabada M. Org. Lett.  2002,  4:  1587 
• 6b Barluenga J. Fañanás FJ. Sanz R. Marcos C. Org. Lett.  2002,  4:  2225 
• 6c Barluenga J. Fañanás FJ. Sanz R. Marcos C. Chem. Eur. J.  2005,  11:  5397 
• 7 For a recent example, see: Hsu D.-S. Hsu P.-Y. Lee Y.-C. Liao C.-C. J. Org. Chem.  2008,  73:  2554 
• For the selective removal of the isopropyl group from aryl isopropyl ethers bearing methoxy groups, see:
• 8a Banwell MG. Flynn BL. Stewart SG. J. Org. Chem.  1998,  63:  9139 
• For the selective removal of the cyclopentyl group from 4-(cyclopentyloxy)-3-methoxy benzaldehyde, see:
• 8b Gajera JM. Gharat LA. Farande AV. Synth. Commun.  2007,  37:  2877 
• 10 [1,2] and [1,4] Wittig rearrangements of allyloxy organometallics such as 12 are observed at higher temperatures. See ref. 6a, and for a review dealing with Wittig rearrangements: Schöllkopf U. Angew. Chem., Int. Ed. Engl.  1970,  9:  763 
• This result could be due to the fact that t-BuLi is dimeric in Et₂O and monomeric in THF. See:
• 12a Bauer W. Winchester WR. Schleyer P.v.R. Organometallics  1987,  6:  2371 
• 12b Bates TF. Clarke MT. Thomas RD. J. Am. Chem. Soc.  1988,  110:  5109 
• 13 See, for instance: Wu J. Beal JL. Doskotch RW. J. Org. Chem.  1980,  45:  208 
• 14 See, for instance: Rosa CP. Kienzler MA. Olson BS. Liang G. Trauner D. Tetrahedron  2007,  63:  6529 

9

The synthesis of the starting allylated ethers 8 was carried out under standard Williamson conditions (treatment of the corresponding phenol with allyl bromide and K₂CO₃ as base in refluxing acetone).

11

Ether 8a could be completely deallylated under the conditions reported by Bailey (t-BuLi, pentane, -78 ˚C to r.t., see ref. 5). We have also checked that 8a could also be deallylated by treatment with t-BuLi in Et₂O at -78 ˚C by using 1 mL of solvent per mmol of 8a.

15

Thioether 13 was prepared from 2-methoxythiophenol by its treatment with NaOH and allyl bromide in water.

16

A small amount of the expected 2-methoxythiophenol could be detected in the crude product of the reaction.

17

Although treatment of 16 with s-BuLi also afforded a mixture of 17 and 18 as main products, in this case 2-methoxymethyl phenol was also obtained as a minor product.