Table 1.

Temperatures and Choice of Activations

entry	activation	solvent	temp [°C]	time [h]	yield [%]a	pyrans	drb	8c [%]
1	Br2	toluened	−78 to rt	12	92	7-Br	6.9:1	0
2		CH2Cl2	−78	2	63	7-Br	6.9:1	0
3		CH2Cl2	−45e	2	73	7-Br	7.0:1	0
4		CH2Cl2	−20e	2	82	7-Br	7.0:1	0
5		CH2Cl2	0e	2	81	7-Br	7.0:1	0
6		CH2Cl2	rte	1	82	7-Br	6.9:1	0
7	NBS	CH2Cl2	−78 to 40	24	NR	7-Br		0
8	NBS + TBABf	CH2Cl2	−45 to rt	6	42	7-Br	1:1.2	0
9	NCS	toluened	−78 to 60	48	NR	7-Clg		0
10	NCS + TBACh	CH2Cl2	−45 to rt	24	54	7-Cl	6.8:1	30i
11	l2j	CH2Cl2	−78 to 40	24	30	7-lk	1:1.1	33l
12	NlS	CH2Cl2	−78 to 40	24	NR	7−1		0
13	lCl	CH2Cl2	−78 to rt	2	70	7-l	6.0:1	0

^aIsolated yields. NR: no reactions.

^bRatios assigned using ¹H and/or ¹³C NMR

^cX = H at C3. See Scheme 2 for the structure.

^dBr₂ was added as a 0.5 M solution in CH₂Cl₂: toluene:CH₂Cl₂ is 3:1.

^eBr₂ was added at −78 °C. The reaction was stirred at the temperature and time indicated after the addition.

^fTBAB: tetra-n-butyl ammonium bromide; NBS:TBAB:6 = 1.5:1.5:1. NBS and TBAB were premixed at −78 °C for 10 min.

^gX = Cl at C3.

^hTBAC: tetra-n-butyl ammonium chloride; NCS:TBAC:6 = 1.5: 1.5:1. NCS and TBAC were premixed at −78 °C for 10 min.

ⁱdr for 8 = 2:1.

^j4 Å MS was used.

^kX = I at C3.

^ldr for 8 = 3:1.