You can’t do this with anonymous objects; field names must be valid identifiers. You could instead use a Dictionary, which Json.Net should serialise just as easily as an anonymous object:
var document = new {
conditions = new Dictionary<string, string>() {
{ "acl", "public-read" },
{ "bucket", "s3-bucketname" },
{ "starts-with", "test/path" }
}
};