The present invention provides for compounds of Formulas I-III, and salts thereof,
wherein n, m, R
1
, R
2
, R
3
, and R
4
have any of the values defined there for in the specification. The compounds of formula I may be used in the synthesis of oligonucleotides such as compounds of formula II (bearing a 3′-thiol) or formula III (bearing a 3′-disulfide).