The invention provides compounds of formula I:
or a salt thereof as described herein. The invention also provides pharmaceutical compositions comprising a compound of formula I, processes for preparing compounds of formula I, intermediates useful for preparing compounds of formula I and therapeutic methods for suppressing an immune response or treating cancer or a hematologic malignancy using compounds of formula I.
Howles; Thorpe; Udall, Journal of the Chemical Society, 1900, vol. 77, p. 944