Table 1.

Specific rotation values observed for diketopiperazines 5a–d.

Cmpd	[α]D 1	Lit.
5a cyclo(l-Trp-l-Arg)	−10.5 (c 0.105, MeOH)	−13 [12]
5b cyclo(d-Trp-d-Arg)	+10.7 (c 0.103, MeOH)
5c cyclo(l-Trp-d-Arg)	+26.0 (c 0.131, MeOH)
5d cyclo(d-Trp-l-Arg)	−26.4 (c 0.421, MeOH)

¹ Acquired in MeOH at the concentration given in g/100 mL.