Lactone-based probes and methods of use thereof

A compound of formula I:

• or a salt thereof, wherein:
• R1 is H, (C1-C6)alkyl, (C1-C6)alkoxyl, aryl, heteroaryl, or 4-8 membered heterocycle;
• wherein the (C1-C6)alkyl is optionally substituted with one or more groups independently selected from halo, —ORa, —CN, —NO2, -oxo-, —N(Ra)2, —CO2Ra, aryl, heteroaryl, or 4-8 membered heterocycle, wherein any aryl, heteroaryl, and 4-8 membered heterocycle is optionally substituted with one or more groups independently selected from halo, —ORa, —CN, —NO2, -oxo-, —N(Ra)2, or —CO2Ra;
• R2 is H or (C1-C6)alkyl;
• R3 is H or (C1-C6)alkyl;
• R4 is H, (C1-C6)alkyl, (C1-C6)alkoxyl, aryl, heteroaryl, or 4-8 membered heterocycle;
• wherein the (C1-C6)alkyl is optionally substituted with one or more groups independently selected from halo, —ORb, —CN, —NO2, -oxo-, —N(Rb)2, —CO2Rb, aryl, heteroaryl, or 4-8 membered heterocycle, wherein any aryl, heteroaryl, and 4-8 membered heterocycle is optionally substituted with one or more groups independently selected from halo, —ORb, —CN, —NO2, -oxo-, —N(Rb)2, or —CO2Rb;
• L1 is absent or (C1-C4)alkylene;
• L2 is absent or a linking group;
• Y is —N3, (C2-C6)alkynyl, 3-8 membered cycloalkyl comprising at least one triple bond, 1,2,4,5-tetrazinyl which is optionally substituted with (C1-C6)alkyl, biotin, or a detectable group;
• each Ra is independently hydrogen or C1-4 alkyl;
• or two Ra groups taken together with the nitrogen to which they are attached form pyrrolidinyl, piperidinyl, piperazinyl or morpholinyl;
• and
• each Rb is independently hydrogen or C1-4 alkyl;
• or two Rb groups taken together with the nitrogen to which they are attached form pyrrolidinyl, piperidinyl, piperazinyl or morpholinyl.